Ohio Sen. JD Vance is prepared for Tuesday's vice-presidential debate with Gov. Tim Walz, Jason Miller, campaign senior adviser for former President Donald Trump, said on Monday.

"JD Vance is great. That's why President Trump picked him," Miller said in a press call previewing the debate hosted by the Trump campaign and the Republican National Committee.

Still, Miller worked to manage expectations, calling Walz a good debater who will be "very polished."

"It's two years in official politics for JD Vance, but two decades in politics for Tim Walz," Miller said, later adding that Walz will "have things memorized. He'll be buttoned up. He'll be very polished."

As Vance gets ready to face Walz, the Ohio senator has turned to Minnesota Rep. Tom Emmer to help him in debate rehearsals by playing Walz, sources familiar with the plans told ABC News.

One of the sources said Emmer was invited to stand in for Walz so that Vance could prepare to take on the governor's folksy personality.

On Sunday, Emmer told "This Week" co-anchor Martha Raddatz that he had been helping Vance get ready for the debate -- pulling from his previous experiences with Walz to help. The two overlapped in the House from 2015 through 2019 before Walz ran for governor of Minnesota.

"I've known Tim probably since he was first elected almost 20 years ago, and I worked with him directly for four years, I spent the last month just going back, all of his old stuff, to get his phrases down, his mannerisms, that sort of thing," Emmer said. "My job was to be able to play Tim Walz so JD Vance knows what he's going to see."

Emmer praised Vance, saying the Ohio senator will do a "great job" on Tuesday night and that "he's got the issues on his side."

Emmer is the third-ranking Republican in the House and serves as the majority whip. Emmer also previously served as the chair of the National Republican Congressional Committee.

Emmer backed Trump for president earlier this year -- despite the fact that the former president called Emmer a "Globalist RINO" who is "totally out-of-touch" with Republican voters, effectively tanking Emmer's speakership bid in October 2023.

Vance's debate preparations have included sessions at his Cincinnati home and online sessions with his team and with Miller, a source told ABC News.

Vance has spent the last month reviewing debate plans, strategies and potential questions, according to a source familiar with the senator's debate preparations.

Vance is expected to paint Walz as too liberal, focusing on the policies he has passed while governor of Minnesota, one of the sources said.

Vance's wife, Usha Vance, a Yale law school graduate and who clerked for U.S. Supreme Court Chief Justice John Roberts, has also been involved in the debate preparation sessions, a source told ABC News.

Those helping Vance prepare have been immersed in learning Walz's debate style by watching videos of his past debates from his previous campaign runs, according to the source.

The source also points to Vance's frequent media interview as helping him prep for the debate, specifically citing his frequent appearances on the Sunday shows and engaging with "contentious network hosts."

Walz's debate preparations are also underway with Transportation Secretary Pete Buttigieg acting as a Vance stand-in during the Walz team's debate rehearsals. Walz has also held policy sessions with his own longtime aides, Biden White House alumni and members of the Harris-Walz campaign team.

The vice presidential debate hosted by CBS News is set to be in New York City on Tuesday, Oct. 1, the network has announced, with both Walz and Vance agreeing to participate. The debate will be moderated by "CBS Evening News" anchor and managing editor Norah O'Donnell and "Face the Nation" moderator and CBS News chief foreign affairs correspondent Margaret Brennan.

ABC News Isabella Murray and Sarah Beth Hensley contributed to this report.