It didn’t take long for Mehmet Oz to invoke his recent endorsement from former President Donald Trump — or for his Republican rivals to try to undercut it.

Four times in the opening 20 minutes of Pennsylvania’s first GOP Senate debate Monday night, the celebrity surgeon used Trump’s support to fend off attacks against his conservative credentials, while his opponents repeatedly accused him of flip-flopping from more moderate or liberal positions.

David McCormick, who is rivaling Oz for the lead in public polling, also faced questions about his former hedge fund’s more than $1 billion in investments in China. But it was the TV star widely known as “Dr. Oz” who took the most criticism.

The event was the first formal, public debate of the Republican primary to include the two front-runners, Oz and McCormick, whose wealth and lavish TV spending have overshadowed the rest of the GOP field. Kathy Barnette, Jeff Bartos, and Carla Sands each used the night to try to boost their standings as the alternative to the two big-spending leaders.

Monday’s debate was also the first major GOP event since Oz won Trump’s coveted endorsement earlier this month. Hours before the debate, Trump announced a May 6 rally with Oz in Westmoreland County.

Here are some key takeaways from the night:

Oz goes all in on Trump

While McCormick has long attacked Oz as “a Hollywood liberal,” Oz had one answer Monday night: Trump. It pointed to the power Oz sees in the endorsement.

Asked about abortion, Oz began by pointing to Trump, even though the surgeon has in the past expressed concerns about some abortion restrictions.

“President Trump endorsed me and quite clearly the first point he made about why I’m a conservative, America First Republican: He said that I am pro-life,” Oz said.

And even before the inevitable attacks came over his past comments on fracking and transgender issues, Oz used Trump as a shield to dismiss them.

“Mr. McCormick approached President Trump with this type of information but was unable to pull the wool over his eyes because he saw right through him,” Oz said.

» READ MORE: Is Mehmet Oz really a conservative? We looked at the Pa. Senate candidate’s record.

Oz has a long trail of public comments that run counter to some of the positions he has taken in the primary, but clearly he’s banking on Trump’s endorsement to wash much of that away and prove his conservative bona fides.

His opponents had to find new ways to undercut that support.

“The reason Mehmet keeps talking about President Trump’s endorsement is because he can’t run on his own positions and his own records,” said McCormick, who had heavily lobbied for Trump’s support. “He’s flip-flopping, and the problem, Doctor, is there’s no miracle cure for flip-flopping.”

Barnette, a conservative commentator, argued that the Make America Great Again approach “does not belong to President Trump” but to the voters, while Sands repeatedly pointed out that Trump appointed her ambassador to Denmark.

But the focus on Oz showed that his rivals see him as their main threat, and his reliance on Trump’s support shows what he thinks will carry the primary.

The ‘carpetbagger’ questions

Three of the five candidates on stage — both front-runners plus Sands — made huge fortunes while living much of their adult lives outside Pennsylvania, including the years immediately before they decided to move back and run for Senate.

That was an immediate flash point.

“When the carpetbaggers lose, you will never see them again,” Barnette said. “And if they should win, you will never see them again.”

Bartos, a Montgomery County real estate developer, described himself as a “lifelong Pennsylvanian” who has raised his two daughters in the state.

“The two tourists who’ve moved here to run, they don’t know Main Street Pennsylvania,” Bartos said. “They haven’t cared to spend time there until they decided to run for office.”

But polls suggest that Oz and McCormick — with their heavy TV spending — aren’t paying a huge price for their many years away from Pennsylvania.

» READ MORE: David McCormick is worth at least $116 million and maybe a lot more, a new report says

Oz, who grew up in Delaware and lived for decades in New Jersey while working in New York, pointed out that he went to medical and business school in Philadelphia and grew up “less than 10 miles from Kennett Square.”

“Pennsylvanians that I speak to are quite clear: They care much more about what I stand for than where I’m from,” Oz said.

McCormick pointed out that he’s a seventh-generation Pennsylvanian who grew up in Bloomsburg and left to attend West Point before returning to run a business in Pittsburgh. (He didn’t mention leaving again to serve in the George W. Bush administration or, in recent years, living in Connecticut while leading the hedge fund Bridgewater Associates.) McCormick, who has owned his family farm for a decade, bought a home in Pittsburgh in November.

“My Pennsylvania roots are deep,” he said. “I’m very connected to my friends where I grew up, and I’m very connected to the people of Pennsylvania.”

Sands, who grew up in the Harrisburg area but spent much of her adult life in Southern California, said she’s an eighth-generation Pennsylvanian and recounted landing her first job at Hershey Park.

Echoing Trump on the issues

Where the candidates did largely agree is on continuing Trump policies on major issues including energy, immigration, and the economy.

Several candidates repeatedly said they would work to remove barriers for gas-drilling companies working to expand hydraulic fracturing, or fracking.

Saying the energy sector is “key to unlocking our economy,” McCormick pointed to a 2014 advice column by Oz that warned about the health risks associated with fracking.

Oz shot back that “dishonest Dave is at it again,” and rattled off statistics about the potential economic impacts of stripping away some regulatory measures in the energy sector. He said the backers of the so-called Green New Deal — congressional legislation President Joe Biden doesn’t support — are “shutting down our ability to harvest natural gas from under our feet here in Pennsylvania.”

While Oz and McCormick bickered over energy, Bartos painted himself as the most friendly to small businesses, which he tied back to immigration, at one point saying “saving Main Street” will take job creation that starts “in many cases with a secure border.”

All five candidates portrayed the Biden administration’s border policies as far too lax, with Oz and McCormick suggesting it was contributing to an influx of drug trafficking and a crisis of overdose deaths.

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Election myths abound

It seemed like a pretty simple question: Is it time for the Republican Party to move on from the 2020 election?

Bartos answered first, saying he’s focused on November and the opportunity to challenge the Biden administration.

It devolved into a recitation of election-denial greatest hits, with several candidates leaning into conspiracy theories or questioning election procedures that were repeatedly upheld by courts.

Sands shouted out a documentary about election fraud by a conservative filmmaker and suggested the election was “stolen by the Zuck bucks,” referring to a grant program by a nonprofit organization partly funded by Mark Zuckerberg that aimed to expand access to mail ballot drop boxes.

McCormick also insinuated the Facebook founder improperly influenced the election, mentioning the “money coming in from Zuckerberg.”

» READ MORE: Inside a hunt for voter fraud on Philly’s Main Line and the conspiracy world that embraced it

Barnette seemed to imply that the 2020 congressional race she lost in the Philadelphia suburbs was tainted by voter fraud, a claim for which there is no evidence. She has repeatedly aligned herself with election conspiracies and those who propagate them.

And Oz said he’s discussed the 2020 election with Trump “and we cannot move on.” He signaled support for continuing to review the election, saying, “We have to be serious about what happened in 2020, and we won’t be able to address that until we can really look under the hood.”