"It's entirely consistent with Sen. McCain's record during his 26 years in Congress where, time and again, he's opposed Medicare," Obama said. "In fact, Sen. McCain has voted against protecting Medicare 40 times."
Campaign officials said the $882 billion estimate was drawn in part from a study by the Center for American Progress, a public policy organization stocked with prominent Democrats.
In response, McCain's campaign issued a statement saying Obama was "simply lying." The statement said the Republican planned to trim spending, but said his plans "do not cut a single benefit."
Ahead in the national polls, Obama made his charge as he campaigned in a traditionally Republican state where he has invested heavily in hopes of collecting 13 electoral votes. He is spending far more on television advertising in Virginia than McCain and has 50 offices statewide. The trip was his seventh here since wrapping up the Democratic nomination in June.
Still, in a state that once boasted the capital of the Confederacy, Obama's campaign indicated it understands the challenge involved in trying to elect the nation's first black president.
Democratic Sen. Jim Webb never mentioned race as he introduced Obama to the predominantly white crowd at the Roanoke Civic Center in the southwestern part of the state. But, he said, "Barack Obama's father was born in Kenya. Barack Obama's mother was born in Kansas by way of Kentucky," he said, adding that Obama would be the "14th president of the United States whose ancestry and whose family line goes back" in the region.
"You can trust him. I trust him," Webb added.
Obama's remarks on Medicare amounted to a new front in the campaign's health care wars, and were aimed at persuading older voters to abandon McCain.
McCain wants to provide tax credits to encourage Americans to purchase private health insurance. To pay for it, he has proposed requiring workers to pay income taxes on the health benefits they now receive tax-free from their employers.
McCain's campaign says additional funding will be required to cover the full cost of the program.
"It turns out, Sen. McCain would pay for part of his plan by making drastic cuts in Medicare $882 billion worth, $882 billion in Medicare cuts to pay for an ill-conceived health care plan, even as Medicare already faces a looming shortfall," Obama said.
"It would mean a cut of more than 20 percent in Medicare benefits next year. If you count on Medicare, it would mean fewer places to get care, and less freedom to chose your own doctors," he added.
Obama said his own proposals for Medicare include "eliminating wasteful subsidies to big HMOs in Medicare, and making sure seniors can access home-based care, and letting Medicare negotiate with drug companies for better prices."
The reference to subsidies referred to the money the government pays to support private alternatives to traditional government-run Medicare, but attempts by some Democrats to eliminate them ran into bipartisan opposition in Congress over the past two years.
Recent studies show the government pays an estimated $112 for Medicare patients in private coverage for every $100 it spends on the traditional program, and that eliminating the difference could save more than $150 billion over a decade.
Critics say the private alternative is wasteful. Supporters argue it often provides benefits such as vision care that are unavailable in government-run Medicare.
Like Obama, numerous other Democrats favor allowing the agency that runs Medicare negotiate directly with pharmaceutical companies, saying that would lead to cheaper prices for prescription medicines.
But the non-partisan Congressional Budget Office said last year the change was unlikely to result in lower prices unless the government decided to limit the drug choices available to seniors like the Veterans Administration does.