An independent campaign from Mr. Trump would splinter the Republican base and all but ensure another four years for Democrats in the White House. Mr. Trump, who has been registered in the past as a Democrat and a Republican, considered running for the Reform Party’s presidential nomination in 2000.
Mr. Trump has long viewed politics through a personal lens, equating disagreements with his policies and tactics to a lack of allegiance to conservatism as a whole. One of the former president’s favored put-downs of opponents inside his party is to dismiss them as “RINOs,” or Republicans in name only.
Even loyalty to Mr. Trump and his personal brand of Republicanism has often not been enough to avoid being crushed by the former president’s dominating style of politics.
In primaries last year, Mr. Trump refused to back campaigns for numerous longtime supporters and former officials in his administration — including Lou Barletta’s campaign for governor of Pennsylvania — over candidates who appeared more likely to win.
Mr. Barletta, a former congressman, was one of Mr. Trump’s earliest supporters in Congress in 2016, but the former president instead endorsed State Senator Doug Mastriano. Mr. Mastriano was leading in the primary polls but lost in the general election to the Democrat, Josh Shapiro, by double digits.
In the 2016 race, Mr. Trump was also initially unwilling to say whether he would back the eventual Republican nominee, a source of deep alarm to party leaders and officials. During a Republican debate in August 2015, Mr. Trump was the only one of 10 candidates onstage who refused to pledge support to the eventual nominee — and the only one not to rule out a third-party bid.
At the time, Mr. Trump said his decision would depend on how the Republican Party treated him.
A month later, he declared he was being treated fairly by the party, and signed a pledge to support the eventual nominee during a private meeting at his Trump Tower office with Reince Priebus, who, at the time, was chairman of the Republican National Committee.
Mr. Trump then hosted a news conference in the lobby of Trump Tower. Mr. Priebus did not attend, despite Mr. Trump’s insistence.
Maggie Haberman contributed reporting.