"The Obama people played this very well," said Tim Blessing, a professor of history and political science at Alvernia University in Pennsylvania.

But the president never seized enough of a lead to become a prohibitive favorite. Frank Newport, editor in chief of the Gallup Poll, noted that neither candidate has been able to build, let alone retain, much of an edge.

Newport could not say with any precision why the numbers trickle up and down. Not even Ryan appears to have made much difference. But he at least gave Romney a jolt of energy.

"He’s articulate, young and aggressive, and he’s obviously competent," Brown said.

And as chairman of the House Budget Committee, the Wisconsin Republican helps press the point that Romney’s chief mission is to revive a struggling economy.

Medicare is likely to remain a key focus. Lyndon Johnson, a Democrat, signed the health care program for seniors and some disabled people into law in July 1965. Obama has vowed to keep the financially ailing system alive and largely intact.

The 2010 federal health care plan would cut at least $700 billion from future anticipated Medicare spending – though it would not affect benefits to seniors – and creates an independent board to recommend cuts.

Ryan and Romney offer a different approach. Ryan’s plan in 2023 would replace Medicare’s guaranteed coverage for new beneficiaries with a payment to seniors called a voucher, which they could then use to buy private coverage, or use traditional Medicare. If the medical costs were higher than the voucher amount, seniors would have to pay the difference.

The tax debate offers another key difference. Romney wants to cut tax rates 20 percent across the board. Ryan would slash those rates even further, changing the current five-rate structure, with a top rate of 35 percent, to one with two rates of 10 percent and 25 percent.

Obama would retain current rates only for individuals earning less than $200,000 and joint filers making less than $250,000. Everyone else would pay pre-Bush-era rates, with a top rate at 39.6 percent. Romney says lower spending, ending some deductions and a revived economy would help pay for his tax cuts.

Add to all of this a contest with an increasingly ugly tone, and the stage is set for a pointed and personal campaign that could remain in doubt until the end.

"It’s been a close race," Newport said. "It’s hard to tell what will happen. Both candidates are hanging in there around 46 percent."