From the very beginning of his presidency, Donald Trump has never really left “campaign mode” — but as the next election gets closer, that approach has turned into a more concrete play for victory in 2020. But Trump is not alone. He has challengers in the 2020 Republican primary, most notably, former Massachusetts Governor Bill Weld, former South Carolina congressman Mark Sanford and former Illinois congressman Joe Walsh.

This campaign is the first time an incumbent president has faced a challenger with name recognition within his own party since 1992, when Republican president George H.W. Bush faced a challenge from more conservative Pat Buchanan — but that wasn’t the only time a sitting President has had to fight for his spot on the ballot.

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, 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.

Both Tyler and Fillmore, who were Whig Party presidents, were denied the nomination because the political battles surrounding slavery: Tyler in 1844, over the annexation of Texas, which he supported but which would upset the balance of free and slave states; Fillmore in 1852 over his support of the Fugitive Slave Act. (Democratic President Franklin Pierce, who ended up winning the 1852 election, also lost his party’s nomination after one term, as many Northern Democrats felt his support for the Kansas-Nebraska Act was too conciliatory to pro-slavery Southerners.) Johnson was the first president to be impeached, in February 1868, so he didn’t get either party’s nomination. And Arthur, who succeeded President James Garfield, was denied the 1884 Republican nomination, though he didn’t actively seek it because he was suffering from kidney disease.