LANCASTER (AP) — Pennsylvania’s thus-far sleepy U.S. Senate race this year could nonetheless play a key role in determining whether Republicans hang onto control of the chamber.

U.S. Rep. Lou Barletta is heavily favored over state Rep. Jim Christiana in Tuesday’s Republican primary election to determine who will challenge Democratic U.S. Sen. Bob Casey in November’s election.

Casey is unopposed for the Democratic nomination.

Barletta is endorsed by the state Republican Party and is a favorite of President Donald Trump, who Barletta endorsed in the 2016 presidential election even before Trump had won Pennsylvania’s primary election.

Trump asked Barletta to run for U.S. Senate — the pair are on good enough terms that Barletta watched this year’s Super Bowl with Trump at the Republican president’s Mar-a-Lago club in Florida — and Barletta remains a staunch supporter.

On Saturday, Barletta’s campaign said it sent out recorded telephone calls of Trump criticizing Casey and reminding hundreds of thousands of registered Republicans why he supports Barletta “fully, strongly, and proudly.”

“Lou is my guy, and I need him in the U.S. Senate,” Trump says on the recording, according to Barletta’s campaign. “Like me, Lou Barletta is strong on the wall, tough on immigration and has already cut your taxes. He was with us all the way. If you, like all of the others, want to keep it going this way, go out and vote for Lou.”

Barletta has run for months as the GOP’s presumptive nominee, targeting his criticism toward Casey.

“He has resisted, rejected and obstructed every single thing that Donald Trump has done,” Barletta told a Lancaster County Republican Party dinner crowd Thursday night.

Christiana focuses his energy on Barletta, but not for Barletta’s support of Trump. Rather, Christiana criticizes Barletta as “saying one thing during a campaign and then turning around doing something completely different in Washington.”

Casey, a fierce Trump critic, is seeking a third six-year term. The 58-year-old Scranton native is the son of the late former governor, Robert P. Casey, and is possibly the best-known political name in Pennsylvania. He has won five statewide elections, served as state treasurer and auditor general, and won his 2012 Senate election by more than 9 percentage points.

Barletta won his House seat during the Republican midterm wave of 2010, catapulted by the attention he received while mayor of the small city of Hazleton for cracking down on immigrants in the country illegally. Hazleton approved Barletta’s measures in 2006, denying permits to businesses that hire people in the country illegally and fining landlords who rent to them, but they were never enforced before the U.S. Supreme Court struck them down in 2014.

Casey is one of 10 Democratic senators who represent a state won by Trump, putting a target on his back for Republicans. The GOP has a slight majority in the U.S. Senate.

For now, Casey holds a large cash advantage over Barletta, $9.9 million to $1.3 million as of April 25. Casey also has midterm political winds at his back: the party of the president nearly always loses seats in Congress in midterm elections.

Barely any outside money has made its way into Pennsylvania thus far to help Casey or Barletta, making it unlikely that Pennsylvanians will see a repeat of the record-breaking $180 million U.S. Senate race in 2016 that Republican Sen. Pat Toomey won by fewer than 2 percentage points.