PHOENIX — Former President Donald J. Trump returns on Saturday to Arizona, a cradle of his political movement, to headline a rally in the desert that will be a striking testament to how he has elevated fringe beliefs and the politicians who spread them — even as other Republicans openly worry that voters will ultimately punish their party for it.

Mr. Trump’s favored candidate for governor, Kari Lake, is a first-time office seeker who has threatened to jail the state’s top elections official. His chosen candidate to replace that elections official, a Democrat, is a state legislator named Mark Finchem, who was with a group of demonstrators outside the Capitol on Jan. 6 as rioters tried to stop the certification of the 2020 election. And one of his most unflinching defenders in Congress is Representative Paul Gosar, who was censured by his colleagues for posting an animated video online that depicted him killing a Democratic congresswoman and assaulting President Biden.

Mr. Trump has invited all three to join him onstage on Saturday for an evening that promises to be full of political revelry and retribution, marking the former president’s unofficial debut in a midterm election year in which he will try to exert a heavy hand.

But as popular as the former president remains with the core of the G.O.P.’s base, his involvement in races from Arizona to Pennsylvania — and his inability to let go of his loss to Mr. Biden — has veteran Republicans in Washington and beyond concerned. They worry that Mr. Trump is imperiling their chances in what should be a highly advantageous political climate, with Democrats deeply divided over their policy agenda and Americans taking a generally pessimistic view of Mr. Biden’s leadership a year into his presidency.

Senator Mitch McConnell, the Republican leader, and other senior party officials have expressed their misgivings in recent days about Mr. Trump’s fixation on the last election, saying that it threatens to alienate the voters they need to win over in the next election in November.

Those worries are particularly acute in Arizona, where the far-right, Trump-endorsed slate of candidates could prove too extreme in a state that moved Democratic in the last election as voters came out in large numbers to oppose Mr. Trump. The myth of widespread voter fraud is animating Arizona campaigns in several races, alarming Republicans who argue that indulging the former president’s misrepresentations and falsehoods about 2020 is jeopardizing the party’s long-term competitiveness.

“I’ve never seen so many Republicans running in a primary for governor, attorney general, Senate,” said Chuck Coughlin, a Republican consultant who has worked on statewide races in Arizona for two decades. “Usually you get two, maybe three. But not five.”

For Republicans who are concerned about Mr. Trump’s influence on candidates they believe are unelectable, the basic math of such crowded primaries is difficult to stomach. A winner could prevail with just a third of the total vote — which makes it more than likely a far-right candidate who is unpalatable to the broader electorate wins the nomination largely on Mr. Trump’s endorsement.

In a general election, Mr. Coughlin said, “If the race is about election integrity, and you’ve got a Trumper and another person who believes in the election, the other person wins the race.”

Conservative activists in Arizona have long supplied Mr. Trump with the energy and ideas that formed the foundation of his political movement. In 2011, when the real estate developer and reality television star was testing the waters for a possible presidential campaign, his interest in the conspiracy theories that claimed former President Barack Obama’s birth certificate was a forgery led him to Arizona Tea Party activists and a state legislator. They were pushing for a state law to require that political candidates produce their birth certificates before qualifying for the ballot. Mr. Trump invited them to Trump Tower.

His attacks on undocumented immigrants helped endear him to Arizona voters who had long supported politicians who vowed to crack down on illegal immigration. And a stretch of the Arizona border with Mexico became home to part of Mr. Trump’s unfinished, signature wall.

More recently, because of Mr. Finchem and other pro-Trump politicians, Arizona has been a hotbed of distortions about what happened in the 2020 election. Allies of the former president demanded an audit in the state’s largest county, insisting that the official outcome had been compromised by fraud. But when the results of the review were released — in a report both commissioned and produced by Trump supporters — it ended up showing that he actually received 261 fewer votes than first thought.

Still, the myth lives on. And those who question it quickly become targets of the former president and his allies. They have attacked two prominent Arizona Republicans — Gov. Doug Ducey and State Attorney General Mark Brnovich for their roles in Arizona’s formal certification of its election results.

Mr. Trump issued a statement on Friday, insisting that if Mr. Ducey decided to run for the United States Senate seat occupied by Mark Kelly, a Democrat, the governor would “never have my endorsement or the support of MAGA Nation!”

Mr. Brnovich is running in that Senate primary, and a Republican political group supporting one of his opponents recently ran an ad accusing the attorney general of “making excuses instead of standing with our president” over the 2020 election.

Few Republicans have been willing to call Mr. Trump out publicly for misleading his supporters in a state where all four Republicans in its House delegation voted to overturn the results of the election when Congress convened to certify on Jan. 6. Mr. Gosar, a featured speaker at Mr. Trump’s rally on Saturday, was the first House member to object that day.

Those who have broken ranks with their party include Stephen Richer, the Maricopa County recorder, who has started a political action committee to support Republicans running for state and local office who accept the validity of the last election. “The Arizona election wasn’t stolen. We Republicans simply had a presidential candidate who lost,” the group declares on its website.

But even those who have resisted going along with Mr. Trump’s false claims have been unable to completely duck the issue when faced with pressure from the president and his supporters.

When a group of 18 Republican attorneys general signed onto a far-fetched lawsuit from their counterpart in Texas that sought to delay the certification of the vote in four battleground states that Mr. Trump lost, Mr. Brnovich did not join his colleagues. He declared at the time that the “rule of law” should prevail over politics. But as a candidate for Senate who still occupies the office of the attorney general, he has investigated claims of fraud at the behest of Trump supporters.

And with state lawmakers reconvening to begin their 2022 session, it appears that 2020 will still be on the minds of many Republicans. Pro-Trump legislators are expected to continue to push for a vote to invalidate the results of the last election. But it will be an uphill battle. They do not yet have enough support in the State Senate, and the governor’s office has said the legislature lacks the power to do what Mr. Trump’s most ardent supporters are demanding.