But in interviews with more than 75 Republicans at every level of the party, elected officials, donors and strategists expressed widespread uncertainty about whether Mr. Trump would be on the ballot in 2020 and little doubt that others in the party are engaged in barely veiled contingency planning.
“They see weakness in this president,” said Senator John McCain, Republican of Arizona. “Look, it’s not a nice business we’re in.”
Mr. Trump changed the rules of intraparty politics last year when he took down some of the leading lights of the Republican Party to seize the nomination. Now a handful of hopefuls are quietly discarding traditions that would have dictated, for instance, the respectful abstention from speaking at Republican dinners in the states that kick off the presidential nomination process.
In most cases, the shadow candidates and their operatives have signaled that they are preparing only in case Mr. Trump is not available in 2020. Most significant, multiple advisers to Mr. Pence have already intimated to party donors that he would plan to run if Mr. Trump did not.
Mr. Kasich has been more defiant: The Ohio governor, who ran unsuccessfully in 2016, has declined to rule out a 2020 campaign in multiple television interviews, and has indicated to associates that he may run again, even if Mr. Trump seeks another term.
Mr. Kasich, who was a sharp critic of the Republicans’ failed attempt to repeal the Affordable Care Act with deep Medicaid cuts, intends to step up his advocacy by convening a series of policy forums, in Ohio and around the country.
“He’ll continue to speak out and lead on health care and on national security issues, trade policy, economic expansion and poverty,” John Weaver, a political adviser of Mr. Kasich’s, said.
In the wider world of conservative Trump opponents, William Kristol, editor at large of The Weekly Standard, said he had begun informal conversations about creating a “Committee Not to Renominate the President.”
“We need to take one shot at liberating the Republican Party from Trump, and conservatism from Trumpism,” Mr. Kristol said.
It may get worse, said Jay Bergman, an Illinois petroleum executive and a leading…