Thomas Hofeller, a Republican strategist developed a "computerized mapping system" for state redistricting. To use that system in every state required knowing citizenship. The citizenship question was proposed for the 2020 census to collect that information. While this might cause an undercount affecting apportionment, that was never its purpose. I recommend a complete reading of the Vox article, quoted in part below, for its discussion on census response rates and undercounts and how the Census Bureau tries to ensure a complete count.
The citizenship question is not directly related to President Trump (though it might provide some benefit); but, rather, it was intended specifically for state redistricting, though presumably it could be used for Congressional redistricting. Such redistricting would last for ten years, thus beyond President Trump's last term.
What was needed to put the strategy into effect was the right, willing administration official and some support.
Thomas Hofeller was a Republican political strategist primarily known for his involvement in gerrymandering electoral district maps favorable for Republicans.
In the early 1970s, Hofeller developed a "computerized mapping system" for the California State Assembly. In the 1980s, he was behind a strategy to increase Republican power in the South by using the 1965 Voting Rights Act to create more majority-black districts and thus pack African-Americans into fewer districts and make it easier for Republican candidates to win the remaining white districts. According to The New York Times, Hofeller's views on skewed maps appeared to be motivated by a desire to strengthen Republican power; during the 1980s, Hofeller opposed Democratic maps that were skewed in favor of Democrats, but later became an advocate for similar maps skewed to favor Republicans.
Vox, The fight over the 2020 census citizenship question, explained, June 12, 2019:
The discovery of hard drives from a Republican operative that included a study on the effect of drawing state legislative districts based on the citizen population pointed to another political advantage Republicans could gain and added to a pile of evidence about the administration’s original intent. With census data on citizenship, the Republican operative, Thomas Hofeller, wrote, conservative states with large immigrant populations could vote to exclude non-citizens from the count they use to draw state legislative districts, consolidating Republican statehouse control.
Strategy versus tactic:
Strategy is a set of choices aimed to provide required environment for performing an action required for achievement of a goal, while tactics are the specific actions, the strategy make the tactical options (in which efficiency can vary based on variables) available for selection.
In other words, tactic is a set of actions with certain requirements, aimed to achieve a goal, and strategy is a set of actions aimed to achieve a goal using the available resources.
The goal for Republicans is Republican control of government. The goal for President Trump is campaign promises.
The strategy was to use federal law as a means to enhance Republican control and thus reduce Democratic influence.
The tactic was to use the 1965 Voting Rights Act as a reason for collecting citizenship data in the 2020 Census.
Why does Trump want a citizenship question on the census?
As the nominal head of the Republican Party, President Trump is expected to use his position to further the goals of the Republican National Committee (RNC). One of those goals is targeted redistricting for which the RNC needs the citizenship question on the census. This is needed for the plan devised by Thomas Hofeller who was a paid consultant to the RNC. While the citizenship question and redistricting were not among President Trump's campaign promises, helping their political party is expected of all presidents.
The point being that President Trump and the Republican Party may have different goals. President Trump has used different tactics, for example, to try to get the wall. Republicans have used different tactics for their goals. In this case, the tactic, if successful, could satisfy their different goals.
A tactic is not the goal it is simply a means to that end.
Another strategy and tactic could be to use the Constitution and an Executive Order to ask the question. (It's a novel approach; I could find no prior case law in my usual sources.)
ARTICLE II, SECTION 3
He shall from time to time give to the Congress Information on the State of the Union, and recommend to their Consideration such Measures as he shall judge necessary and expedient; ...
Claim that the census question is necessary to recommend to Congress such Measures as he shall judge necessary and expedient and this cannot be done without asking the question.
Different tactic same goal; both tactics using the citizenship question.
What are the stated reasons Trump and the Republicans want this question on the Census?
Trump considering executive order on citizenship question for census:
"You need it for many reasons. Number one, you need it for Congress, for districting. You need it for appropriations: Where are the funds going, how many people are there, are they citizens, are they not citizens? You need it for many reasons,” Trump said.
The Republicans (the RNC) wants the question for redistricting, as mentioned earlier. President Trump, who is planning on being president for four more years, wants more Republicans in Congress. Both need more Republican control to meet their goals.
Even though the plan has been made public, raising the citizenship issue by pressing for the inclusion of the question in the census will provide talking points for the 2020 election campaigns. That is, Democrats may be asked to justify why representation for illegal immigrants is more important than representation for citizens. It may also spur smaller, Republican controlled states to push for a Consitutional convention to add an amendment making citizenship the basis for apportionment, as mentioned below.
Whether or not the question is actually included on the census, it appears to be a win for Republicans; simply because it's in the news and being discussed.
The Census Bureau can supply citizenship data to states, without asking the citizenship question.
In Major Move, Census Bureau Offers Up Citizenship Data For Redistricting, December 28, 2018:
In what could be a major change for voting rights and the distribution of political power between urban and rural areas, the Census Bureau signaled Friday that it is willing to work with state and local officials charged with drawing voting districts if they want citizenship data for the redistricting process.
The Federal Courts are no longer involved in most redistricting.
Supreme Court Rules Partisan Gerrymandering Is Beyond The Reach Of Federal Courts, June 27, 2019:
In a 5-4 decision along traditional conservative-liberal ideological lines, the Supreme Court ruled that partisan redistricting is a political question — not reviewable by federal courts — and that those courts can't judge if extreme gerrymandering violates the Constitution.
The ruling puts the onus on the legislative branch, and on individual states, to police redistricting efforts.
Two more wins for Republicans.
And, also for Republicans, it appears the goal is to reduce the number of Democrats in the House of Representatives. But the current attempt is not the first.
The Congressional Reseach Service provided a report, Constitutionality of Excluding Aliens from the Census for Apportionment and Redistricting Purposes, April 13, 2012.
From time to time, Congress has considered legislation that would exclude all aliens or prevent only unauthorized aliens from being included in the census for purposes of apportioning House seats among the states. Such legislation would have either amended the Census Clause of the Constitution or enacted or amended federal census statutes. Although such legislation has yet to be introduced in the 112th Congress, in the 111th Congress, legislation was introduced that used both approaches. The Fairness in Representation Act would have statutorily excluded aliens from the population count for apportionment purposes (H.R. 3797 and S. 1688). Under the above analysis, it would not appear to be constitutionally sufficient for Congress to amend the federal census statutes in such manner. Meanwhile, H.J.Res. 11 would take the other approach and amend the Constitution so that only U.S. citizens would be counted in the apportionment calculation.
H.J.Res.11, 111th Congrss, was introduced January 7, 2009.
H.J.Res.11 - Proposing an amendment to the Constitution of the United States to provide that Representatives shall be apportioned among the several States according to their respective numbers, counting the number of persons in each State who are citizens of the United States.
This resolution had 27 Republican cosponsors and one Democratic cosponsor.
What reasons have Democratic Party politicians and progressive pundits expressed for opposing it?
One belief is that an undercount will occur, thus reducing the amount (Federal dollars) sent to communities. Another is that an undercount will affect apportionment.
Democrats worry the real purpose of having the census count citizens is to change how seats in Congress are allocated
Democrats celebrate announcement on citizenship census question:
Critics have pushed back on the White House's efforts to ensure its inclusion for months, arguing it had the potential to cause noncitizens and anyone else in their households to skip filling out the question or partaking in the census altogether, which could lead to an inaccurate count.
Senate Minority Leader Charles Schumer (D-N.Y.) released a statement shortly after the decision was announced, calling it "a victory for our democracy."
"The exclusion of the citizenship question from the census is a victory for our democracy and for fair representation of all communities in this country," Schumer said. "The Trump administration’s politically-motivated efforts to undermine the Constitution in this instance were so reprehensible that even the conservative Supreme Court couldn’t let them get away with it. Democrats in Congress will be watching the Trump administration like a hawk to ensure there is no wrong-doing throughout this process and that every single person is counted."
Other Senate Democrats also heaped on the praise, including Minority Whip Dick Durbin (D-Illi.), Brian Schatz (D-Hawaii) and Mark Warner (D-Va.).
And Rep. Judy Chu (D-Calif.) said the announcement was a "victory for democracy," arguing the change could have impacted the amount of money appropriated for key priorities in certain areas of the country.
"Victory for democracy! The Constitution requires a count of ALL people. Now we need your help to spread the word and encourage everybody in your community to participate in #Census2020. An accurate count impacts representation & federal $$$ for housing, transpo, health & more," she said on Twitter.
Why hasn't this question been on previous Censuses?
It has been asked before.
FACT CHECK: Has Citizenship Been A Standard Census Question?:
The census has been conducted every decade since 1790 to get a national head count used most critically to decide the distribution of congressional representation. At first it was conducted by U.S. marshals, but later surveys were sent to most American households, with census workers helping those who didn't promptly return their surveys.
The last time a citizenship question was among the census questions for all U.S. households was in 1950. That form asked where each person was born and in a follow-up question asked, "If foreign born — Is he naturalized?"
In 1960, there was no such question about citizenship, only about place of birth.
Starting in 1970, questions about citizenship were included in the long-form questionnaire but not the short form. For instance, in 2000, those who received the long form were asked, "Is this person a CITIZEN of the United States?"
Later, the census added the American Community Survey, conducted every year and sent to 3.5 million households. It began being fully implemented in 2005. It asks many of the same questions as the census long-form surveys from 1970 to 2000, including the citizenship question.
But if the 2020 census form does ultimately ask about citizenship status, it will be the first time the U.S. census has directly asked for the citizenship status of every person living in every household.