Has a sitting US president ever been denied their party's nomination for a second term?

Question

Has there ever been an instance when a US president didn't get a second term because they failed to get his party's nomination? From the looks of it, there's nothing that could prevent a sitting US president from getting re-elected other than losing or voluntarily not participating in a general election (typically, due to an unsatisfactory economy). Still, the party that holds the White House, as I understand it, always carries out primary elections across the country.

Do I understand correctly that a sitting president has never been denied renomination and that the process is merely, in practice, a formality?

Answer 1

In terms of not achieving the nomination of their party due to losing the primary contests directly, no. However, there have been times when the incumbent president seeking re-election has pulled out of the contest early, for example in 1968 when Lyndon B. Johnson pulled out of the race after winning the first primary in New Hampshire by only 7 percent - thereby technically not losing - or when the incumbent came very close to losing in the primary contests overall.

As the modern primary system in the US presidential race has only been in place since the 1970s, an innovation in part due to the aforementioned 1968 contest, I'll only consider cases since then. The most notable in my opinion took place in the 1976 Republican primaries, when Gerald Ford was challenged for the nomination by Ronald Reagan. Ford won the nomination after a close race, by 1,121 delegates to Reagan's 1078.

The incumbent president being challenged in the primary has become increasingly rare in recent decades; the current challenger to Trump for the nomination, Bill Weld, is the first serious challenger since Patrick Buchanan challenged Bush in 1992. This is partly because conventional wisdom dictates that a strong challenger in the primaries is correlated with the President losing the re-election campaign. It is questionable whether Bill Weld meets the definition of a strong challenger.

Answer 2

No incumbent president has lost his primary race, but you have to keep in mind that primaries are a 20th century invention basically.

The 1976 campaign season was the year in which primaries started to matter more than ever before, and is considered the closest a sitting President has come to losing his party’s nomination in modern history. President Gerald Ford — who was elected to the House of Representatives, but became first Vice President then President thanks to the resignations of Spiro Agnew and Richard Nixon — was vulnerable, thanks especially his unpopular decision to pardon Nixon. The nomination was still up for grabs when the Republican National Convention started in Kansas City, Mo., but Ford eked out win the day before the convention was supposed to end.

On the other hand

Before primary elections became the dominant way to pick a nominee, party leaders were more able to either shut down challengers or smoothly pass the nomination to someone else. Notably, four incumbents who were denied the nomination in the 19th century — John Tyler, [Millard Fillmore,] Andrew Johnson and Chester A. Arthur — had been Vice Presidents who rose to the Presidency following the deaths of their predecessors, perhaps suggesting they’d never won their parties’ full support in the first place.

Answer 3

Since the ratification of the Constitution, five incumbent U.S. presidents who were eligible for another term and who actively sought the presidential nomination¹ have been denied it, all of them long before the primary era when being nominated for president only required a few hundred votes rather than millions:

• John Tyler (W/I²) in 1844 (the Whigs hated him by 1844, and it wasn't technically even his party anymore, since they'd expelled him from it back in September 1841 for voting against it).
• Millard Fillmore (W) in 1852 (the northern Whigs dropped him like a hot potato over the Fugitive Slave Act, causing him to narrowly lose the nomination to Winfield Scott).
• Franklin Pierce (D) in 1856 (two words: "Bleeding Kansas").
• Andrew Johnson (D) in 1868 (even the Democrats realised that the recent impeachee was a millstone around their collective neck by then).
• Chester A. Arthur (R) in 1884 (in poor health, likely wouldn't have been up to the task of being president another four years).

Four of those five (Tyler, Fillmore, Johnson, and Arthur), as noted by the source given in @Fizz's answer, were elected as vice presidents, and succeeded to the presidency upon a presidential death. Only one of the five, Pierce, was elected as president only to be denied the nomination the next time around.

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¹: Thus excluding incumbents who bowed out very early in the race, such as Truman in 1952 and LBJ in 1968.

²: Although Tyler was elected vice president (and succeeded to the presidency upon the death of William H. Harrison) on the Whig ticket, the party expelled him in late 1841 for voting against it.

Answer 4

This doesn't fully fit the bill but is an interesting example nonetheless. Remember that the two-term limit wasn't actually enshrined in the constitution until the 22nd Amendment was passed in 1951 after Franklin D. Roosevelt was elected a total of four times between 1932 and 1944. It was only considered good form prior to said amendment in part because Washington had proposed and set the precedent. Thus, this case from the first half of the 20th century should qualify to a certain extent nonetheless.

The 26th President was Theodore Roosevelt who was elected on a Republican Party ticket. Initially elected as Vice President in 1900 he ascended to the Presidency upon his predecessor's assassination in 1901. He was re-elected on a Republican ticket in 1904. In 1908, he did not stand for re-election, instead supporting Taft as his successor.

During Taft's term as president Roosevelt became increasingly unhappy with Taft's administration. This culminated in Roosevelt contesting the nomination again in 1912, hoping to win a third term as president on the Republican ticket. Roosevelt won a plurality of delegates in primary elections; however, at the time not all states held primaries and the majority of delegates from these states went to Taft. Ultimately, the RNC chose Taft as their candidate for the 1912 presidential election.

Roosevelt was not satisfied and instead set up his own party, the Progressive (Bull Moose) Party to contest the election. He was mildly successful, winning second place in the popular vote and the Electoral College over Taft. However, the Democratic candidate Wilson secured a plurality of the popular vote and a landslide Electoral College victory, probably due to the Republican vote being split between Taft and Roosevelt. A crude analysis suggests the election would have gone the other way with only one candidate on the Republican side.