Pennsylvania picks its candidates Tuesday for the crucial US midterm elections in a contest that has gripped the political establishment -- with a Democratic frontrunner suffering an 11th-hour stroke and a far-right Republican outsider surging into contention.
It is the most consequential day so far of the primary season, with hopefuls in North Carolina, Oregon, Kentucky and Idaho vying to be their party's representative in November's congressional and state ballots.
All eyes are on Pennsylvania's Republican Senate contest, however, which looked for months like a toss-up between hedge fund tycoon David McCormick and TV surgeon Mehmet Oz, who has former president Donald Trump's endorsement.
But the race has been rocked in the final days by a come-from-behind surge from a long-shot insurgent Kathy Barnette, who has drawn practically level with her big-spending rivals.
Barnette had been so far behind that the early frontrunners spent little time digging into her background until recent days, when a series of anti-Islam and homophobic statements were uncovered.
Oz, who would be the first Muslim Republican senator if elected, zeroed in on a 2015 social media post in which Barnette said that "pedophilia is a cornerstone of Islam."
She has falsely denied posting the slur and told Fox News on Sunday that some of her inflammatory posts were aimed at starting a conversation, rather than "full thoughts."
She is also facing questions over her appearance at the "Stop the Steal" rally that turned into the 2021 US Capitol riot, and spreading Trump's false claims of election fraud.
- 'Full recovery' -
The ex-president's support for Oz will serve as a new test of his sway over Republicans, after a clean sweep of successful endorsements in the first three state primaries but failure of a high-profile candidate in Nebraska a week ago.
A Fox News poll released this week showed Barnette with support from 19 percent of respondents, McCormick with 20 percent and Oz with 22 percent.
The anti-abortion Barnette has seen her popularity swell among conservatives after speaking powerfully about her mother being raped at age 11 and giving birth to her at just 12.
But Republican leaders worry that she will prove unable to expand her appeal beyond the Republican base, costing the party an eminently winnable Senate seat in November.
Donna Patterson, head of political science at Delaware State University, told AFP Barnette's "unprecedented rise" would likely shut out McCormick, leaving Oz as her main rival.
"Frankly, things have shifted so quickly in the past 10 days that it is difficult to predict a winner," she added.
Trump said in a statement last week however that Barnette would "never be able to win the general election against the radical left Democrats."
In the race for Pennsylvania governor, Trump has the party fretting all over again after he endorsed far-right candidate Doug Mastriano, a state senator who has a narrow polling lead but is considered by many to be too out of the mainstream to win in the midterms.
The Pennsylvania primary took another turn, this time on the Democratic side, as Lieutenant Governor John Fetterman, the overwhelming favorite for the party's Senate primary, suffered a stroke on Friday.
The tattooed, six-foot-nine-inch (2.05-meter) liberal said he had suffered no cognitive damage and was expected to make a "full recovery" but he remains hospitalized.
According to the most recent Franklin and Marshall University poll, Fetterman leads Marine veteran and moderate US congressman Conor Lamb by nearly 40 percentage points.