As Tim Kaine prepares to walk onto the biggest political stage of his life at the vice presidential debate on Tuesday, insiders and a former rival say one of his strongest assets will be the same thing that has helped him through a 22-year political career: his likability.

He seems to know that too. As he left Mass on Sunday in Richmond, Virginia, Hillary Clinton's running mate told reporters that on Tuesday he just has to "be myself."

"I'm calm," Kaine said of his upcoming debate with GOP vice presidential nominee Mike Pence. "Things work out the way they are supposed to."

But behind the calm is intense preparation.

Longtime Democratic campaign aide Mo Elleithee has informally given Kaine thoughts and advice as he prepares for this week's debate at Longwood University in Farmville, Virginia. Elleithee worked closely with Kaine during his Senate and gubernatorial campaigns.

Elleithee said Kaine usually begins his debate prep by working alone, reading and absorbing information, before he engages in mock debates. He said Kaine thinks about the debate format and the best way to use that format to make his arguments.

"He doesn't have a pair of lucky debate socks or anything crazy," Elleithee said. "He takes his prep seriously, spends a lot of time thinking about how he wants to frame the arguments he wants to make."

Kaine spent the past few days prepping in Raleigh, North Carolina, and his hometown of Richmond, Virginia. Washington, D.C., attorney Robert Barnett is playing Pence in Kaine's mock debates. Members of Clinton's prep team as well as Kaine's staffers have been involved in his debate preparations.

Kaine has described this debate as a unique challenge.

"It's a different kind of a debate for me because I have done debates where it's, at the end of it, 'Please vote for Tim Kaine.' If I talk too much about Tim Kaine during my debate, I'm wasting my time. It would not be a good way to use my time, and it is about two visions for the country. And it's about a Clinton presidency or a [Donald] Trump presidency, so I'm more than a surrogate, because I'm on the ticket. But I am not the main event, so I'm in an in-between space. And I've not done that before," Kaine told reporters aboard his campaign plane on Sept. 22.

Elleithee predicted that Pence and Kaine will spend more time discussing their running mates' records than their own. Elleithee described it as a "proxy debate for the top of the ticket."

"These two guys can really get into a conversation about the direction each ticket wants to take the country, but there is this other layer of stuff the Clinton campaign has been trying to drive home. This notion that not only are Donald Trump's policies bad, but he's temperamentally unfit. I think Kaine is very comfortable driving both those messages, and I'm sure Pence would rather focus on the former rather than the latter, but you can't escape the latter," Elleithee said.

Kaine and Pence will likely have to defend the vulnerabilities of the candidates at the top of the ticket too.

For Kaine, that will likely mean defending Clinton's use of a private email server when she was secretary of state. For Pence, that may mean defending Trump's leaked tax return and his comments last week about women like Alicia Machado, a former Miss Universe whom Trump has attacked.

"Mike Pence kind of has one job in this debate, and that is to change the trajectory of the race, particularly after the last week, and I'm curious to see how he does that, because the biggest problem of Trump's trajectory the last week is the perception people have of Donald Trump, and the only person who can fix that is Donald Trump," Elleithee said.

Kaine told his traveling press corps that part of his prep involved connecting Pence's record to Trump's.

"It's not knowing another fact, but it is about thinking hard about the material, thinking hard about Pence's record and also what Pence's record would say about the guy who chose him, since it really is more about Donald Trump than it is about Gov. Pence," Kaine told reporters Sept. 22.

Along with Kaine's prep comes a sense of self that insiders say doesn’t waver, even with the nerves that come with a big stage.

"I've never seen a debate-stage Tim Kaine that looked different than Tim Kaine any other day. He is who he is," Elleithee said.

That sense of self can be frustrating for those standing on a debate stage with Kaine. Jerry Kilgore, a former attorney general in Virginia, ran against Kaine during Virginia's gubernatorial race in 2005.

Kilgore described Kaine on a debate stage as "just being honest and straightforward," even when defending or explaining vulnerable parts of his record.

"He can look into that camera and absolutely be believable, and unless his opponent in the debate is armed with facts, he's going to get away with it," Kilgore said.

Kilgore also pointed to Kaine's background as a Harvard-trained lawyer.

"He is Harvard educated. He's an attorney, and he does this sort of — he's used to, like, speaking to the jury, and the jury in this case is the voters, and he will be able to pull that off unless he's challenged," Kilgore said.

Kilgore, a Republican, has not talked to Pence but offered this assessment.

"It will be frustrating for Gov. Pence to look over and listen to Sen. Kaine," Kilgore said. "He will play to the audience that is in front of him, which is the American people, trying to lure in the independent voters."

Kilgore and Elleithee predicted that Kaine will not engage in personal attacks on the debate stage.