5 Storylines to Watch During the VP Debate

RICHMOND, Virginia— -- Today marks a moment in the presidential campaign that is not squarely focused on the presidential hopefuls but, rather, on their running mates.

Sen. Tim Kaine and Gov. Mike Pence are facing off at the vice presidential debate at Longwood University in Farmville, Virginia, tonight, and there are several important storylines to keep in mind.

What Happens When Both Candidates Prepare

While there was much speculation about how much or little Hillary Clinton and Donald Trump prepared for the first debate, at Hofstra University last week, Kaine and Pence have been prepping for their time in the limelight.

Pence has been using Wisconsin Gov. Scott Walker as his Kaine stand-in during mock debates, and D.C. power lawyer Bob Barnett, who played Bernie Sanders in Clinton's practice debates, has been playing Pence during Kaine's mock debates.

Kaine has been reading briefing materials during campaign air travel and taking time off the trail to dedicate to debate preparation.

Defending the Top of the Ticket

Even though Kaine and Pence have their own political histories and records, the nature of the presidential race means that they will likely spend much of their time onstage tonight discussing the tops of their tickets.

Both vice presidential candidates have spent time with their running mates on the trail, and while neither Clinton nor Trump will be in Virginia tomorrow, their presence — and policies — will certainly be felt on the debate stage at Longwood University.

Kaine Hoping for Home State Advantage

Virginia is leaning Democratic in recent polls, according to ABC News' electoral map, and Kaine is undoubtedly banking on his decades-long history in the state to help him onstage tonight.

Before being picked as Clinton's running mate, Kaine, who is the junior senator for Virginia, served as its governor, its lieutenant governor and the mayor of its capital, Richmond.

This may be Kaine's first time in a spotlight this bright, but his aides stress that he was a longtime lawyer and litigator and that he has been preparing for the debate as if he were about to make a case in court, laying out his arguments one at a time.

Playing Off the Polls

The latest national presidential poll, released by CBS on Monday, has Clinton at 45 percent to Trump's 41 percent, within the poll's margin of error.

Clinton's slight lead comes after a CBS poll in mid-September had her and Trump tied at 42 percent.

With 35 days left until the election, the influence of polls is being felt more dramatically than before.

Comparisons With Past VP Debates

The dynamics of the presidential race differ every cycle, and while Trump and Clinton bring star power to their debates, it was the VP debate that got more attention in the 2008 race.

The prime-time showdown between Sarah Palin, John McCain's running mate, and Joe Biden, Barack Obama's running mate, was the most watched debate of that election cycle, with 69.9 million viewers, according to Nielsen.

The Kaine-Pence face-off is not expected to bring in that kind of number, but it will be interesting to see how they fare, given that the first Clinton-Trump debate had an estimated 84 million viewers — a record for presidential debates.

Watch FULL LIVE COVERAGE of the second presidential debate, co-moderated by ABC News’ Martha Raddatz, at 9 p.m. ET on Sunday, Oct. 9. Coverage and analysis of the debate will begin on ABCNews.com/Live at 7 p.m. ET.