DENVER - With polls showing likely voters across the country nearly evenly split over the presidential contenders, Republican Mitt Romney and President Obama hunkered down in Western swing states on Monday to start cramming for their first presidential debate scheduled for Wednesday at the University of Denver.

Obama made his last predebate campaign appearance in Las Vegas on Sunday night before traveling to Henderson, Nev., to meet with his debate prep team. He planned to remain at a lakeside resort hotel in suburban Las Vegas through Tuesday to prepare for his first face-to-face meeting with Romney.

Romney, meanwhile, arrived in Denver on Monday for a rally at Wings Over the Rockies Air and Space Museum. He was scheduled to leave the campaign trail after that rally and spend his remaining time preparing to take on the incumbent president.

For both candidates, the stakes in the debate are high. It's the first of three scheduled debates between the two presidential contenders and the one that focuses on domestic issues, including the struggling economy, which voters now rank as their top concern.

Most national polls now show Obama with a 2 percentage point lead over Romney, which is within the margins of error of those polls, meaning the two are statistically tied. More importantly, polls in battleground states show Obama with a slightly stronger lead. The latest polls in Ohio, for instance, give the president a 4-point lead over Romney.

"This is a very close race," Kevin Madden, a top Romney campaign aide, told reporters Monday.

Madden and other Romney aides said Monday their candidate will use the remaining weeks of the campaign to "lay out the important contrast" between Romney and Obama.

Romney's most direct opportunity to distinguish himself from Obama will come with Wednesday's debate, when he will have a chance to put Obama on the defensive over the nation's slow economic recovery and persistently high unemployment rate.

Like most incumbent presidents entering a debate, Obama is looking to turn in a performance that simply does no harm to his standing.

As they hunker down to cram for the debate, Obama and Romney aren't just poring over policy papers and statistics; they're both running through full practice debates with surrogates standing in for their opponent.

Sen. Rob Portman, R-Ohio, will be standing in for Obama to help Romney prepare. Portman also played Obama to help Sen. John McCain prepare for the debates with Obama in 2008.

Sen. John Kerry, D-Mass., the 2004 Democratic presidential nominee, will be the stand-in for Romney during Obama's practice runs.

The days before a presidential debate are all about "practice, practice, practice," Kenneth Bickers, a political science professor at the University of Colorado at Boulder, told The Washington Examiner.

"It's somewhat like what law schools do when they have mock trials," he said. "The other side is instructed to be as tough as possible. They would rather lose in a mock debate than in a real one."