With conservative Senate candidate Kathy Barnette surging in the latest polls in Pennsylvania’s GOP Senate race, threatening the Trump-backed Mehmet Oz, the former president took aim at Barnette five days before the Keystone State primary.

"Kathy Barnette will never be able to win the General Election against the Radical Left Democrats," former President Donald Trump said on social media on Thursday. "She has many things in her past which have not been properly explained or vetted, but if she is able to do so, she will have a wonderful future in the Republican Party—and I will be behind her all the way."

The former president, who traveled to Pennsylvania last Friday to hold a rally with Oz, added that "Dr. Oz is the only one who will be able to easily defeat the Crazed, Lunatic Democrat in Pennsylvania. A vote for anyone else in the Primary is a vote against Victory in the Fall!"

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A Fox News poll conducted May 3-7 and released on Tuesday was the latest public opinion survey to indicate that Barnette’s support among likely GOP primary voters in Pennsylvania has jumped in recent weeks, putting her into a virtual tie with front-runners and Dave McCormick in the contest to succeed retiring Republican Sen. Pat Toomey. The Keystone state race is one of a handful across the country that will likely determine if the GOP wins back the Senate majority in November’s midterm elections.

(Fox News )

Oz, the cardiac surgeon, author and well-known celebrity physician who until the launch of his Senate campaign late last year was host of TV’s popular "Dr. Oz Show," stood at 22% in the Fox News poll. McCormick, a former hedge fund executive, West Point graduate, Gulf War combat veteran and Treasury Department official in former President George W. Bush's administration, registered at 20% in the survey. Barnette, veteran and conservative political commentator, was at 19%. Nineteen percent were supporting other candidates, and 18% remained undecided.

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The Oz and McCormick campaigns and outside super PACs backing the two contenders have spent tens of millions of dollars to run TV, digital and radio ads attacking each other over their conservative credentials and over key issues since the beginning of the year. 

McCormick has slammed Oz as a "RINO" [Republican in name only] and is spotlighting comments Oz made in 2019 when he said he had concerns over an Alabama measure that would have led to a near total ban on abortion. And McCormick’s questioned Oz – who holds duel citizenship with Turkey – over his connections to the Turkish government. Oz has repeatedly attacked McCormick for investments in China made by the hedge fund behemoth he steered until stepping down to run for the Senate.

Kathy Barnette, Mehmet Oz, moderator Greta Van Susteren, Carla Sands, David McCormick, and Jeff Bartos, (left to right) pose for photo before they take part in a debate for Pennsylvania U.S. Senate Republican candidates, Wednesday, May 4, 2022, in Grove City, Pa. (AP Photo/Keith Srakocic)

Kathy Barnette, Mehmet Oz, moderator Greta Van Susteren, Carla Sands, David McCormick, and Jeff Bartos, (left to right) pose for photo before they take part in a debate for Pennsylvania U.S. Senate Republican candidates, Wednesday, May 4, 2022, in Grove City, Pa. (AP Photo/Keith Srakocic) (AP Photo/Keith Srakocic)

Without the resources available to Oz and McCormick, Barnette by comparison has spent a miniscule amount to run ads.

She told Fox News on Wednesday that "I’ve been driving over 1,500 miles a week, going all across the commonwealth. I’ve spent a tremendous amount of time with the voters here. People really feel unnerved."

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And she said her message is getting out, as attested in the latest polls. Barnette said that voters "now know that they have a better option. They don’t have to hold their noses and settle this time."

Barnette told Fox News Digital she didn’t get a chance to meet with Trump before his endorsement of Oz last month.

"I’m very grateful for what President Trump has done when he was in office. I certainly believe his policies are superior to the policies of Biden. And yet, he’s not Jesus. He gets to be wrong, and I believe he’s wrong on this endorsement and I believe Pennsylvanians, the people who live here, recognize that and that’s what’s being reflected in the polls," she said.

Trump, nearly 16 months removed from the White House, remains the most popular and influential politician in the GOP, as he continues to play a kingmaker’s role, endorsing scores of candidates up and down the ballot in this year’s midterm elections, and as he repeatedly teases another presidential run in 2024.

Pennsylvania Senate candidate Mehmet Oz, left, accompanied by former President Donald Trump speaks at a campaign rally in Greensburg, Pa., Friday, May 6, 2022. (AP Photo/Gene J. Puskar)

Pennsylvania Senate candidate Mehmet Oz, left, accompanied by former President Donald Trump speaks at a campaign rally in Greensburg, Pa., Friday, May 6, 2022. (AP Photo/Gene J. Puskar) (AP Photo/Gene J. Puskar)

Just over a week ago, Trump’s last-minute endorsement of JD Vance in Ohio’s jam-packed, combustible, and expensive Republican Senate primary was instrumental in boosting the former hedge fund executive and best-selling author to victory. 

After the candidates Trump endorsed in Ohio and Indiana primaries held on May 3 were victorious, Trump boasted in a Fox News interview that "I was 22 and 0…. I won every race."

But Trump’s clout over the GOP took a symbolic hit after the candidate he was backing in Nebraska’s crowded and divisive Republican gubernatorial primary lost. The former president suffered a rare proxy defeat in the race, as Charles Herbster — the candidate he had endorsed and traveled to Nebraska to hold a rally to support — came up short.

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It’s likely another Trump-backed candidate will go down to defeat next Tuesday. In Idaho, the former president is supporting far-right Lt. Gov. Janice McGeachin, who is one of seven primary challengers running against incumbent Republican Gov. Brad Little, whom polling suggests is the overwhelming front-runner.

Now, the possibility of Oz losing on the same night has some in Trump world nervous.

"This race in Pennsylvania is much closer than Trump ever thought it would be. And Tuesday is going to be a major blow if Oz isn't the nominee. And that is noticed in Trump world," an adviser to the former president, who asked to remain anonymous to speak more freely, told Fox News.

Barnette’s been criticized by some Republicans as too extreme to win a statewide general election in a purple state like Pennsylvania, and for being an untested open book. But the candidate insisted to Fox News that "there are no bones that are going to fall out of my closet."

Barnette’s bare-bones campaign is getting some last minute help from a group with deep pockets.

Republican Senate candidate Kathy Barnette of Pennsylvania stars in a new commercial by the outside group Club for Growth, which endorsed Barnette a week ahead of the May 17 primary. 

Republican Senate candidate Kathy Barnette of Pennsylvania stars in a new commercial by the outside group Club for Growth, which endorsed Barnette a week ahead of the May 17 primary.  (Club for Growth Action)

The Club for Growth, the pro-business and anti-tax group that’s a major spender in Republican primaries, recently endorsed Barnette and on Wednesday announced that it has dished out $2 million to run an ad statewide in Pennsylvania that highlights the candidate’s biography.

Earlier this week, Barnette landed the endorsement of the Susan B. Anthony List (SBA List), a leading anti-abortion outside group.

Trump’s team says that the former president plans to follow up last week’s rally with a tele-rally ahead of Tuesday’s primary, and to use robocalls to urge GOP voters to support Oz.

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And Oz, in an interview Wednesday night on Fox News’ "Hannity," questioned Barnette’s electability in the general election in the swing state.

"She is a mystery person, we don't know much about her, and we have to be able to learn, and she's not willing to share," Oz told Fox News’ Sean Hannity.

Fox News' Courtney De George contributed to this report