DENVER - Republican presidential nominee Mitt Romney's tax cut proposals consumed the opening moments in his first debate versus President Barack Obama, as the two candidates sparred over whether the math behind the Republican presidential nominee's plans matched his rhetoric.

The president said Romney would cut taxes by $5 trillion, but hadn't accounted for how he would pay for them. For his part, Romney argued that Obama would not live up to his promise to balance the budget, and argued tax hikes would devastate an anemic economy.

The initial focus on pocketbook issues matches an election in which the economy has been the undisputed top issue for voters. Either candidate would be forced to contend almost immediately with these issues upon taking office in January, when a series of tax hikes and automatic spending cuts are set to spring into place.

"My number one principle is there will be no tax cut that adds to the deficit," Romney said here on the campus of the University of Denver.

Romney asserted his tax plan would spur job creation and help balance the budget, but the president insisted that these proposals simply do not add up.

"The fact is that if you are lowering the rates the way you described, governor, then it is not possible to come up with enough deductions and loopholes that only affect high-income individuals to avoid either raising the deficit or burdening the middle class," Obama said. "It's math. It's arithmetic."

The first half of the debate tied together discussion of taxes, economic growth and the mounting national debt. Romney and Obama largely both stuck to arguments they had litigated so far in the campaign. So intense, though, was the focus on these issues that both the president and Romney blew past predetermined time limits for topics and admonitions by moderator Jim Lehrer.

Still, neither seemed to land the kind of signature blow versus the other that would seem to change the arc of the campaign.

Obama opened the debate on a personal note, recognizing his wedding anniversary with first lady Michelle Obama, who was here in Denver.

Romney also struck a softer note, recounting stories of how voters had approached him and his wife, Ann, at rallies to plead for their assistance.

"The answer is yes, we can help. But it's going to take a different path," he said.

Romney spent much of the day holed up in his hotel resting and preparing for the debate. Both he and Obama toured the debate site this afternoon; the president traveled to Denver this afternoon from Henderson, Nev., where he spent the earlier part of this week practicing.

GOP presidential candidate Mitt Romney steps out of turn to point out errors he feels President Barack Obama made in describing his tax plan.

Obama enters the debate with a slight advantage over Romney, both nationwide and in key swing states, according to the latest NBC News-Wall Street Journal polls. That raises the pressure for Romney, the former Massachusetts governor, to make up ground versus Obama tonight.

The debate, which is moderated by Jim Lehrer of PBS, features an open-ended format set to maximize the direct sparring between Obama and Romney. The focus of Wednesday's debate is domestic policy, though the recent outbreak of violence - particularly in Libya - could mean that foreign policy is also featured in tonight's showdown.