He has long held that the true measure of a man is his TV ratings. So perhaps it came as no surprise when Donald Trump endorsed a celebrity doctor for a US Senate seat in Pennsylvania.

“They liked him for a long time,” Trump said of Mehmet Oz at a rally in Pennsylvania last week. “That’s like a poll. You know, when you’re in television for 18 years, that’s like a poll. That means people like you.”

Trump stunned his own party by his decision to back Oz, who is struggling in the real polls and far from certain to win the Republican primary.

It was one among dozens of risky bets placed by Trump on extreme candidates. The upcoming primaries – votes in states and districts to decide which Republicans will take on Democrats in November’s midterm elections – are shaping up to be a referendum on his dominance of the party.

Next month could be pivotal. Defeat for Oz by David McCormick in Pennsylvania on 17 May, followed by defeat for Trump-backed David Perdue against incumbent governor Brian Kemp in Georgia a week later, could deal a huge blow to Trump’s status as party kingmaker.

Larry Jacobs, director of the Center for the Study of Politics and Governance at the University of Minnesota, said: “Donald Trump is like a reckless gambler that’s gone into a casino and put his stack of money on one number. Right now the roulette wheel is turning and, if he’s wrong on a number of these, you’re going to see increasing defiance.

“It’s almost certain that the growing sentiment among Republican leadership that Trump’s day has come and gone will be reinforced this year. He’s put his political capital on the line in so many races. A more seasoned politician would have been a little more judicious, a little more careful on these close races.”

A string of primary losses for Trump’s picks could also puncture the aura of inevitability around him as party standard bearer in the race for the White House in 2024, encouraging potential rivals such as Ron DeSantis, the governor of Florida.

Jacobs added: “There are definitely some Republicans looking at the presidential nomination who are ready to take on Trump, particularly if they see him as weakening. That is the orientation of a lot of the Republican leaders. They would like to see Trump quietly drift off into the past. Like so much about Trump, he’s refusing to go along and wants to still be a player.”

Never before has a US president left office only to continue barnstorming the country with campaign rallies and insert himself so aggressively into congressional elections. Why Trump is so willing to jeopardise his brand – and how he would respond to being given a bloody nose by Republican voters – remains a matter of conjecture.

One evident motive is to install loyalists who pass the litmus test of supporting his false claim that the 2020 presidential election was stolen by Joe Biden. Trump is laying the groundwork for purveyors of “the big lie” to take control of election machinery across the country.

Allan Lichtman, a distinguished professor of history at American University in Washington, said: “He’s interested in shaping the party, endorsing those who he thinks support him and his approach to politics, and he’s also trying to put into place in some key swing states like Michigan, Arizona and Georgia folks that he thinks can help him steal the next election if he runs.”

There are also financial incentives. Trump’s Save America group, responsible for countless fundraising events and emails, netted a massive $124m between November 2020 and March 2022 while spending only about $14m, or about 11%, to support midterm candidates, according to an analysis by the Reuters news agency.

Wendy Schiller, a political science professor at Brown University in Providence, Rhode Island, said: “This has become a business for Trump. As his other businesses either get sold or dry up or are subject to lawsuits or criminal investigation, he’s still continuing to raise a lot of money and he gets to support his lifestyle through all of these activities.”

She added: “Second, he’s trying to build local bases of support in swing states. If he does decide to run, these are all people who will be part of county and state political organisations. They’ll be people who are going to vote in those primaries in the Republican party, so the more contacts he makes at the local level, the better positioned he will be.”

A procession of Republican aspirants have beaten a path to Trump’s luxury Mar-a-Lago estate in Palm Beach, Florida, in the hope of joining the anointed ones. They run the gamut from shoo-ins to too-close-to-call to genuine underdogs.

On Friday evening, Trump announced his endorsement of JD Vance, author of the memoir Hillbilly Elegy, for a fiercely competitive Senate primary in Ohio.

“Like some others, JD Vance may have said some not so great things about me in the past, but he gets it now, and I have seen that in spades,” the ex-president explained.

He is also all-in for Sarah Palin, the former governor of Alaska and vice-presidential nominee, who is far from guaranteed to win a vacant House seat in her home state. “Sarah shocked many when she endorsed me very early in 2016, and we won big,” Trump stated. “Now, it’s my turn!”

But perhaps the climactic primary battle will come in Wyoming in August when congresswoman Liz Cheney, who has come to personify the party’s anti-Trump resistance, is challenged for a House seat by pro-Trump Harriet Hageman. Cheney has huge name recognition in the state and is raising vast sums of money.

Some of Trump’s endorsements have already backfired. In the Pennsylvania Senate race – potentially critical to determining which party controls the chamber – he initially backed Sean Parnell, only for the candidate to drop out amid spousal abuse allegations.

Trump’s subsequent decision to support Oz, reportedly encouraged by Trump’s wife, Melania, and Fox News host Sean Hannity, carries liabilities of its own. The host of the syndicated The Dr Oz Show is described by critics as a snake oil salesman. In 2014 he admitted to Congress that some of the products promoted on his show lacked “scientific muster”.

Another setback occurred in Alabama, where Trump retracted his endorsement of congressman Mo Brooks in a Senate race, claiming that Brooks “made a horrible mistake” when he told supporters to put the 2020 election behind them. Most observers suspect the real reason was that polls show Brooks heading for defeat.

Trump’s win-loss record is sure to be studied hard by pundits. Yet if past is prologue, it would not be hard to imagine him dismissing defeats in Pennsylvania, Georgia, Wyoming and elsewhere as the fault of weak candidates or rigged elections, while claiming primary victories as his own.

Frank Luntz, a pollster and political consultant, said, “I don’t think it’s risky for him because he doesn’t acknowledge losing: if they win, they win because of him; if they lose, they lost because of their own failure.”

He added: “He’s not as popular today as he was a year ago. The sheen is not as bright but he’s still the most impactful Republican by far and his endorsement does mean something. I understand why candidates really want it, but they have to think about it: what gets you the nomination in Pennsylvania will cost you the election.”

Trump has been written off countless times before. But at February’s Conservative Political Action Conference, he received more votes in a straw poll for the 2024 nomination than all other Republicans combined.

So would a sprinkling of primary defeats truly break the fever?

Reed Galen, cofounder of the Lincoln Project, an anti-Trump group, said: “Some people will say that; more people might even want to believe it. The chattering class of both parties in Washington DC, starting with [Senator Mitch] McConnell on the Republican side, will read those tea leaves that way.

“But it doesn’t make it true because, ultimately, Trump making an endorsement of someone versus Trump being on the ballot – the majority of those primary voters are going to come back to him if he decides to run again.”

DeSantis and other would-be contenders should therefore not leap to conclusions, Galen added.

“I would tend to agree with the idea that there would be a weakness perceived by potential challengers in 2024,” Galen said. “It may embolden them. I do not believe that equation adds up to them beating him in a primary.”