As California Democrats try to keep President Trump off the state’s Republican primary ballot next year, Republicans in other states are pondering plans to cancel or modify their own 2020 presidential nominating contests and essentially affirm the party’s support for Mr. Trump before voters head to the polls.

A new California law requiring candidates to disclose five years of tax returns has raised fresh legal and constitutional questions on ballot access, while nascent proposals to scale back Republican nominating contests in states such as South Carolina and Nevada have drawn criticism from William F. Weld, the most prominent Republican challenging Mr. Trump in 2020.

Shawn Steel, a Republican National Committee member from California, called his state’s law a “political farce” and predicted it would be struck down “by Christmas, if not sooner.”

He said there is merit to simply allowing voters to express their will and that “competition’s always good,” denting the notion of state parties moving to quash dissent and save Mr. Trump from having to dispatch any primary challengers.

“I’m not worried about Trump in any way. … A lot of Republicans that were not enthusiastic for him three years ago — and there was a lot in California — they’re all on board because his production has been beyond anybody’s expectation,” Mr. Steel said. “So there is no real primary threat in any way that I can see, and my answer is, ‘Who cares?’ Trump’s going to come out looking good no matter what.”

The Trump campaign and the Republican National Committee sued soon after Gov. Gavin Newsom, a Democrat, signed the tax return ballot measure into law last month. The lawsuit argues that the requirement infringes on Mr. Trump’s free speech rights.

Mr. Trump, citing an ongoing IRS audit, has broken with long-standing precedent since his 2016 campaign by declining to release his tax returns. He is fighting a separate legal battle with congressional Democrats, who have sued to try to get their hands on the president’s tax information.

Harmeet Dhillon, a Republican National Committee member whose law firm is representing the RNC in its case against California, said the implications of the state law stretch beyond the presidential contest.

“If the president isn’t there, there will be a lot of Republicans who say, ‘I’m not going to bother to vote. I’m boycotting voting this year,’ ” Ms. Dhillon said. “Which means that in potential congressional races they potentially end up in Democrat-on-Democrat races because of our top-two system.”

Mr. Newsom has defended the law by saying the U.S. Constitution leaves it to the states to determine how their presidential electors are chosen.

Rocky De La Fuente, a perennial candidate challenging Mr. Trump for the 2020 Republican nomination, also sued over the California law — even as he said he plans to release his past five years of tax returns voluntarily.

“If voters want a candidate to release their tax returns, voters are free to withhold their vote from candidates who do not,” Mr. De La Fuente said. “All Republicans must stand united in demanding that state officials be held to account when they threaten fundamental rights guaranteed under the U.S. Constitution.”

Regardless of the court outcome, the law likely won’t have much practical effect on Mr. Trump’s status as the all-but-assured Republican presidential nominee next year — though it would be notable if an incumbent president fails to get his name on the primary ballot in the country’s most populous state.

Still, Republicans in other early presidential states are weighing plans to modify or even cancel their presidential nominating contests next year and declaring their support for Mr. Trump before voters have a chance to express their choices in a primary or caucus.

In South Carolina, the party’s executive committee will decide next month whether to hold a presidential preference primary, said Cindy Costa, a Republican National Committee member from the state, which is famous for holding the first-in-the-South presidential primary.

“Democrats didn’t hold one when they had the White House in 2012 or 1996, and Republicans didn’t hold one in ‘04 or ‘84, so there is a strong precedent that the party that holds the presidency doesn’t normally have a primary,” Ms. Costa said.

Republican parties in Nevada and Kansas are also weighing plans to modify or cancel their caucuses next year.

The Nevada Republican Party is slated to decide next month whether to simply have the state central committee decide on committing delegates to Mr. Trump. The party said the move could allow them to divert resources elsewhere.

Mr. Weld, a former Massachusetts governor, said the president is trying to turn the Republican Party “into yet another of his private clubs.”

“We don’t elect presidents by acclamation in America,” Mr. Weld said in a statement to The Washington Times. “The idea that any incumbent, especially this one, should be allowed to bypass facing voters in a primary or party caucus is an insult to the Republican Party and to the millions of voters who would be disenfranchised.”

Though Mr. Weld is the most prominent Republican challenging Mr. Trump for the nomination, scores of lesser-known candidates routinely file with the Federal Election Commission to run for president every cycle and could be affected by the changes.

Mr. Trump’s campaign said it is taking no position on whether state parties should opt out of primary contests and direct the money elsewhere.

“Regardless, the President will dominate whomever has the wrong-headed idea to challenge him in whatever contest is put in front of him,” said Sarah Matthews, a spokeswoman for the president’s reelection campaign.

States and parties cited cost savings as part of their rationale for changing practices in past election years, said Josh Putnam, an analyst on the primary process and editor of the blog Frontloading HQ.

“This time around, certainly Trump is popular among Republicans, as polls continually tell us,” Mr. Putnam said. “What’s a little bit different this time around is [that] Trump is broadly unpopular among the entire electorate, and that may be driving what Bill Weld’s responding to there.”

The RNC passed a resolution supporting Mr. Trump this year after members toyed with the idea of taking more far-reaching steps to block potential primary challengers.

Solomon Yue, a Republican National Committee member from Oregon who helped draft the resolution that was approved, said the parties in individual states and territories should have the right to determine how they want to run their nominating contests.

“With that in mind, I am very comfortable because you have to trust the people to do the right thing,” Mr. Yue said.

Ms. Dhillon likewise said other states can do whatever they want but she doesn’t think it is right to “disenfranchise” voters.

“Any sort of caucus system is less ‘small d democratic’ than allowing voters to vote [on] a primary ballot,” she said. “When you allow voters to vote, they also come and vote for other candidates and for or against ballot measures, and that’s a right. Republicans have a right to have enthusiasm for who’s on the ballot and have people turn out and vote.”

As Ms. Costa indicated, both parties have histories of trying to do what they can to shore up incumbent presidents in their reelection campaigns and avoid major intraparty challenges.

In 2003, Arizona Gov. Janet Napolitano, a Democrat, vetoed Republican-authored legislation that would have canceled the state’s 2004 primary during President George W. Bush’s reelection campaign. She said the state “can well afford the price of democracy.”

This cycle, Republicans in New Hampshire — the site of the first-in-the-nation presidential primary that helped fuel Mr. Trump’s rampage toward the party’s nomination in 2016 — also considered changing rules that say the state party should remain neutral in primaries.

But they ultimately abandoned the effort amid complaints that the change could be seen as tipping the scales in Mr. Trump’s favor.

“Since when do Republicans oppose competition?” Steve Duprey, a Republican National Committee member from New Hampshire, wrote this year in the New Hampshire Union Leader. “If the party starts favoring incumbents over primary challengers, then what is the case for having the first-in-the-nation primary? Who would come here to compete in a rigged system when Iowa and South Carolina offer fair contests?”

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