Barnette's quick rise has Republicans reeling over potential upset and what it could mean for November

Republicans are watching conservative candidate Kathy Barnette's sudden rise in their party's upcoming Pennsylvania US Senate primary with a mix of unease, wonder and dread.

Barnette's late surge in the closely watched contest has turned what was broadly seen as a two-person race between celebrity doctor Mehmet Oz and former hedge fund manager Dave McCormick into a chaotic three-person affair, with Barnette seen as a wild card who has grassroots momentum mere days before the Tuesday contest.

The unexpected development has caused even the most influential Republicans to panic, including former President Donald Trump himself, who attempted to quash Barnette's campaign by claiming in a statement Thursday that she is not a viable general election candidate.

"Kathy Barnette will never be able to win the General Election against the Radical Left Democrats," Trump said.

Though Trump acknowledged that Barnette, who he described as "not properly explained or vetted," is poised to "have a wonderful future in the Republican Party," he reaffirmed his support for Oz and encouraged GOP primary voters to stick with his chosen candidate.

"A vote for anyone else in the Primary is a vote against Victory in the Fall!" he said.

At a campaign stop Thursday evening, Barnette brushed aside the blistering assessment and smiled when asked about the pointed words from Trump, telling reporters: "I look forward to working with the President."

Asked if the whirling criticism is a sign that she's on the rise, Barnette said, "I would agree."

But like the former President, other Republicans are worried about the myriad unknowns surrounding Barnette, who until this week was barely a household name in Pennsylvania, let alone a Republican candidate garnering national attention.

"She is a giant walking question mark," said one Republican operative working on Senate races. "There has been almost no vetting of her. ... There is a lot that we don't know about her, including basic biographical details."

Oz told Fox host Sean Hannity on Wednesday that Barnette was a "mystery person."

"I don't see any way, any scenario under which she can win a general election," he told Hannity, who was among those pushing Trump behind the scenes to endorse Oz earlier this spring.

One person close to Trump said the former President saw the Oz-Hannity interview as "too heavy-handed" and thought Oz's attacks on Barnette would only make her stock rise further.