President Trump is investing big political capital in Tuesday's Senate primary in Ohio.

In a robocall being placed to Republican voters Monday night, Trump emphasizes his endorsement of Rep. Jim Renacci in the five-candidate contest.

"Tomorrow is Election Day, and I need your vote for my good friend, Jim Renacci," Trump says in the recorded call, the audio of which was shared with BuzzFeed News. "Jim is my guy in Ohio. I’ve endorsed Jim in tomorrow’s Republican primary because he’s already helping me to make America great again."

Renacci, given his experience in the House, has been viewed as the frontrunner in the Republican primary. But polls have shown more than half of primary voters are undecided. A recent survey from Baldwin Wallace University measured Renacci's support at 25%. His toughest rival, investment banker and political unknown Mike Gibbons, has had money to advertise on TV and was at 11% in that poll. Renacci also is independently wealthy and has contributed heavily to his campaign.

The winner of the primary will face Democratic Sen. Sherrod Brown in the fall. Brown is seen as a potential presidential candidate in 2020 if he wins a new term.

"Liberal Democrat Sherrod Brown is Chuck Schumer and Nancy Pelosi’s guy," Trump says in the call, referring to the Democratic leaders in the Senate and House. "He does what they tell him to do. So we need Jim Renacci. The only Republican that’s going to beat Sherrod Brown is Jim, and that’s why I give him my full endorsement."

Renacci has styled himself as a Trump-aligned candidate since initially mounting a bid for governor last year. Trump reportedly encouraged him to switch to the Senate race after the early frontrunner, Ohio State Treasurer Josh Mandel, dropped out of the race. In the past two weeks, Trump has tweeted a Renacci endorsement and appeared with him at an official White House event to discuss taxes in Cleveland.

It's a marked contrast from how Trump is approaching the other two big Senate primaries Tuesday.

In West Virginia, Trump tweeted his opposition to Don Blankenship, a coal baron who was found guilty of engaging in a conspiracy to violate mine safety regulations, but he has not indicated a preference for either of the other two main Republican hopefuls. Many are worried will Blankenship will prevail, absent a strong signal of the president's favorite.

In Indiana, Trump has been quiet, though the three Republican Senate candidates have all presented themselves as the best candidate to represent his agenda.