TLDR: Trump is making in-roads with conservatives, had very little support from the smallish liberal group, (so erosion there was less important), and is the default choice among moderates because the Democratic party has tacked leftward significantly.
The second largest ideological group in America is conservatives. Most remember that Trump was a life-long New York Democrat, only recently aligned with the GOP. He's long been an advocate for gay rights, abortion rights (seems to be more conservative now), trade protectionism (still does), and a few other policy positions traditionally held by Democrats. This Johnny-come-lately approach didn't allow him to maximize his appeal among conservatives.
According to exit polling, Trump won 81 percent [among conservatives], but amazingly Clinton won 16 percent (3 percent going elsewhere or not responding).
Combined, Trump failed to secure roughly one-fifth of conservatives — America’s second largest ideological group — in 2016. This will not happen again. In 2016, Trump was an unknown to conservatives; since taking office, his policies should have removed any conservative doubts. On taxes, immigration, judicial nominations, foreign policy, the economy and social policy, it is hard to imagine conservatives being unhappy.
And unhappy they are not. Republican support, as predicted, has increased to 85% as of March 2019.
Trump's support among liberals was never that great to begin with, but the number of liberals is much smaller relatively to conservatives. So it's less important. From the Hill Link:
In 2016, he won 10 percent of liberals. Do not expect a repetition.
However, liberals are America’s smallest ideological group. So, netting the two out, picking up twice as big a percentage from a larger group, is a great trade for Trump.
How about the moderates? Trump in 2016 was able to appeal to them dramatically.
Trump won 206 counties that had supported Obama in 2008 and 2012, which were heavily concentrated in the Midwestern states that propelled him to an Electoral College victory.
“I think Donald Trump actually represented a triumph of centrism,” says GOP consultant Brad Todd. “A lot of analysts never thought you could run on a platform that was socially conservative and fiscally moderate.”
So Trump can be seen as a 2016 variant of the moderate in that sense. Maybe more. How does the Democratic field line up to win moderates? Poorly. Same source.
Democrats Move Further Left
The Democratic Party seems to be engaged in an effort to see how far an American party can move to the left. Prominent presidential candidates and other Democratic politicians are variously talking about preserving and expanding abortion rights, talking up a single-payer health-care plan known as Medicare for All, promising to abolish U.S. Immigration and Customs Enforcement (ICE), and seeking to impose severe energy restrictions as part of a Green New Deal.
Such proposals may excite the party’s progressive wing, but they threaten to put off moderate voters who would otherwise be willing to vote against Trump.
Still, the party’s large and wide-open presidential contest thus far has been a contest to see who can move farthest to the left. That may change, but for now progressives appear to be convinced of the correctness and popularity of their positions, spurning any candidate who veers toward the center or hints at cooperating with the other party.
Progressives nowadays like to deride moderates as wanting only “some” global warming or offering health care to most but not all Americans. The traditional way of finding the center -- working out compromises with political opponents -- has become politically radioactive.
Does this radicalization of the Democrats help Trump electorally among moderates? Most assuredly. The moderates if they have a home will be with Trump. Back to the Hill Article:
...according to Real Clear Politics’ average of national polls, Democrats’ left candidates have a combined 59.2 percent support. It is impossible to see Democrats not nominating a candidate from this rapidly growing majority, and with super delegate rules now changed, there is no brake on Democrats’ going far left: Even should proclivity not lead there, necessity will.
Such a far-left Democrat nominee becomes Trump’s safety valve on moderate support. For moderates, a far-left nominee becomes the ultimate unknown, but policy positions on spending, taxes, social issues, immigration and foreign policy — all are likely to give them significant concerns. In contrast, after four years Trump will be “known,” having survived four years he will at worst benefit from “choosing the devil we know over the devil we don’t.”
And then you have the incumbency bump.
Since 1916, elected incumbents have averaged a 3.4 percent increase in their popular vote margin when seeking a second term.