Former President Donald Trump is coming to Pennsylvania on Friday with a roster of conservative stars to try to push Mehmet Oz over the finish line in his contentious primary campaign for U.S. Senate.

Trump is headlining a rally in Western Pennsylvania as the nationally watched contest heads into its final 10 days before the May 17 primary, with the celebrity surgeon known as “Dr. Oz” locked in a close contest with former hedge fund CEO David McCormick.

The race is among the most crucial in the country, one of a handful likely to decide control of the Senate. The incumbent Republican, Sen. Pat Toomey, is retiring, and both parties have made the race to replace him a priority.

The event will be packed with celebrities of the right, including J.D. Vance, the Trump-backed Senate candidate who just won a hard-fought GOP primary in Ohio; election conspiracist and My Pillow CEO Mike Lindell; and Dinesh D’Souza, who just released a new movie claiming massive wrongdoing in the 2020 presidential election, despite numerous independent reviews that have affirmed the legitimacy of the results.

How to watch: The event will be streamed live on C-SPAN’s website.

Who: Former President Donald Trump and Pennsylvania Republican Senate candidate Mehmet Oz.

When: The rally is set to begin at 5 p.m., with Trump scheduled to speak at 8.

Where: Westmoreland Fairgrounds in Greensburg, Pa.

Will Trump endorse in the GOP primary for governor?

Trump also arrives amid swirling speculation about whether he’ll get further involved in Pennsylvania’s gubernatorial primary, in which several candidates have sought his support.

Trump has issued a scathing anti-endorsement against one candidate, former U.S. Attorney Bill McSwain, but has not picked a favorite among the rest of the field, and it’s unclear if he will.

A test of Trump’s strength

Trump’s high-profile endorsement of Oz, like his backing for Vance, has emerged as a test of his power to shape the GOP even after leaving office.

Almost every Republican running in the Senate or governor’s race has sought out Trump’s support, showing that at least they believe the former president remains an influential figure in the GOP.

» READ MORE: Who is Mehmet Oz, the celebrity surgeon running for Senate?

McCormick, despite missing out after a personal meeting with Trump, has continued to lay claim to being an “America First” candidate, as has Kathy Barnette, a conservative commentator running third in public polls — and increasingly close to the two front-runners.

Vance’s victory in a crowded Ohio primary this week was widely interpreted as a major victory for Trump. The author who once sharply criticized the former president shot to the front of the field after securing his support.

Trump’s influence in Pennsylvania

The results so far in Pennsylvania have appeared less decisive.

Oz appears to have enjoyed at least some bump from the early April announcement, but public polls suggest it’s still close enough that either he, McCormick, or Barnette could win. A large number of GOP voters appear undecided, even after tens of millions of dollars of TV spending.

Privately, Republican insiders largely say that while Trump hasn’t sealed the deal for Oz, his endorsement could be meaningful in a race that may come down to a few percentage points. McCormick allies argue that Oz’s past statements on issues like guns and abortion — and the avalanche of attack ads against him — have made him unpalatable to a large segment of Republican voters.

» READ MORE: Who is David McCormick, the former hedge fund CEO running for Senate?

McCormick allies have increasingly rolled out attacks this week questioning Oz’s ties to Turkey, the country where he holds dual citizenship. Oz has said he would renounce his Turkish citizenship if elected to the Senate.

Oz has used Trump’s endorsement as a political shield against attacks on his past stands, arguing that the former president’s support shows he can be trusted to stand up for conservative views.