Oklahoma voters cast ballots Tuesday in primary contests for an open U.S. Senate seat and the governor's office in a reliably conservative state where President Donald Trump's influence as a party leader faces fresh scrutiny before the November midterms.

Trump's early endorsement of Republican U.S. Rep. Kevin Hern for the Senate position vacated by Homeland Security Secretary Markwayne Mullin discouraged other major contenders from entering the race in a state that has not sent a Democrat to the Senate since 1990.

A stronger measure of Trump's sway, which has largely held firm in GOP primaries this cycle, could emerge from the crowded contest to replace term-limited Gov. Kevin Stitt.

Trump backed former state Sen. Mike Mazzei last month, entering late into a field that features several well-known Oklahoma Republicans including Attorney General Gentner Drummond, former House Speaker Charles McCall and former public safety director Chip Keating.

The crowded field raises the chance of an August runoff if no candidate clears 50 percent of the vote.

In this solidly Republican state the two open races for top offices have sparked intense competition.

Mullin departed the Senate for a cabinet post after Kristi Noem was dismissed by the president in March amid growing concerns about her performance leading the Department of Homeland Security. Republican Alan Armstrong, an energy executive, currently holds the seat on an interim basis but state rules bar him from running for a full term.

Stitt has served as governor since 2019 and is barred from seeking another term by limits on consecutive service. Trump has appeared eager for Stitt to leave office.

While leading the National Governors Association Stitt clashed with Trump earlier this year over invitations to White House events tied to the group's annual meeting. The dispute prompted Trump to label Stitt a RINO on social media.