"Was John McCain a socialist back in 2000," when he opposed President Bush's proposals? Obama asked at a news conference.
"It's not a very plausible argument," he said of the late-campaign allegations launched daily by McCain and Republican running mate.
Obama made his remarks at a news conference after meeting with national security advisers.
Regardless of the winner of the Nov. 4 election, he said a smooth transition is essential "so that others don't take advantage of us."
He also sidestepped a question of whether he would attend a summit of world leaders the Bush administration has called for Nov. 15 to discuss the global economic crisis.
"We have one president at a time," he said.
The Democratic presidential candidate was asked about a comment by his own running mate, Sen., that Obama could expect to be tested early in his term.
He said the Delaware senator has occasionally engaged in "rhetorical flourishes," but the essential point was that the new president could expect to be challenged no matter who wins.
He said President Bush has pursued "bluster and unilateralism and ideology" over the past eight years, "but we're about creating partnerships around the world to solve problems."
McCain-Palin campaign spokesman Tucker Bounds responded, "It's s not leadership for Barack Obama to promise to be straight with Americans, only to dismiss serious statements and concern from his own running mate as simple rhetorical flourishes. Joe Biden guaranteed a generated international crisis if Barack Obama is elected, and a smile-for-the-cameras press conference isn't going to mitigate the risk of an Obama presidency."
Obama denied the meeting had been called because of political damage stemming from Biden's remarks. He said the foreign policy and national security experts attending have busy schedules so arrangements for the meeting began two weeks ago to assess major foreign developments during the campaign.
With 13 days remaining in the race, most polls show Obama leading in his bid to become the nation's first black president. He was campaigning in Virginia on Wednesday as he kept to an itinerary devoted almost exclusively to states that voted Republican in 2004, an indication of his confidence about the state of the race.
McCain launched a new attack over the weekend, saying Obama's plan to provide a $500 tax credit would include even those who pay no taxes and "convert the IRS into a giant welfare agency, redistributing massive amounts of wealth at the direction of politicians in Washington."
Obama's response at the news conference did not address the $500 tax break. Instead, he said that overall, he wants to reverse the cuts that went to the wealthiest Americans when Bush's plan was enacted in 2001 and use the revenue to give tax cuts to workers who make less than $250,000 a year.
Mr. Bush's reductions provided across-the-board cuts for all income levels, and McCain opposed them at the time, because they were not accompanied by spending cuts.
The Obama campaign e-mailed an excerpt from a McCain statement on the Bush tax cuts in 2001 in which McCain said, "I cannot in good conscience support a tax cut in which so many of the benefits go to the most fortunate among us at the expense of middle class Americans who most need tax relief." The Democratic e-mail also cited a McCain statement from the 2000 campaign that "I really believe, that when you are -- reach a certain level of comfort, there's nothing wrong with paying somewhat more."
Now McCain wants to extend those Bush tax cuts and argues that anything else amounts to a tax increase.
Obama arrived in Virginia on Tuesday night after spending two full days campaigning in another GOP state, Florida.
There, he criticized McCain for offering little more than "willful ignorance, wishful thinking, outdated ideology" to an economy in crisis.
With the chairman of the Federal Reserve and even Bush now indicating support for more economic stimulus spending by Washington, momentum is building for Congress to pass a second package after the election, an idea Obama has encouraged. But McCain has remained cool, saying only that he wants to keep his options open.
At boisterous Miami rally with his wife, Michelle, Obama seized on that, as well as a report that a top McCain economic adviser said the Arizona senator prefers to first evaluate the impact $700 billion financial rescue plan passed earlier this month.
"I've got news for Sen. McCain: Hardworking families who've been hard hit by this economic crisis folks who can't pay their mortgages or their medical bills or send their kids to college they can't afford to wait and see. They can't afford to go to the back of the line behind CEOs and Wall Street banks," Obama told a crowd of more than 30,000 that filled a waterside park.