DAYTON, Ohio — The campaign for president pivoted sharply to character and temperament yesterday as Democrat Barack Obama accused his Republican counterpart of "erratic behavior" and John McCain offered his most public — if still elliptical — criticism of Obama's acquaintance with a one-time domestic bomber.

With the free-falling stock market as a backdrop, Obama sought to use McCain's newest economic proposal — a mortgage bailout plan he announced in Tuesday's presidential debate — to suggest in his sharpest language yet that McCain is unfit to be president.

He criticized a change the Arizona senator made to the mortgage plan that would give a break to lenders that made bad loans.

"So banks wouldn't take a loss, but taxpayers would take a loss," Obama explained, characterizing the switch as "just the latest in a series of shifting positions … this is the kind of erratic behavior we've been seeing out of Senator McCain."

Each candidate has tried to characterize the other as the riskier choice for voters, and McCain's effort yesterday to portray Obama as iffy presidential timber took a page from the 1960s. Campaigning with running mate Sarah Palin at a town-hall-style event in Waukesha, Wis., McCain was asked by an attendee about Obama and "the people that he has hung with."

Without mentioning the name of former Weather Underground member William Ayers, McCain alluded to him as "an old, washed-up terrorist" and said that "we need to know the full extent of the relationship because of whether Senator Obama is telling the truth to the American people or not."

McCain also launched an Internet ad linking Obama and Ayers. The two live near each other in Chicago, and in the mid-1990s Ayers, now an education professor, introduced Obama at a political event at his home. The Illinois senator was 8 years old when Ayers and his colleagues planted bombs to protest the Vietnam War, and it is not clear when Obama learned of Ayers' past behavior. The two are not close, and Obama has criticized the bombings as "detestable."

The moves by Obama and McCain in Middle America suggested the outlines of the campaign with 26 days to go before Election Day: Obama, riding a surge in national and bellwether-state polls, was campaigning in southwestern Ohio. He was using the economy as a cudgel against McCain.

McCain, for his part, was campaigning as the underdog in Wisconsin, where he was trying to stoke concerns about Obama's background and relative lack of national experience to peel away voters.

More bluntly than he has in the past, McCain conceded yesterday what political analysts have suggested for weeks: that his campaign is in trouble, and time for a shift in fortunes is diminishing.

"In case you missed it, this is about the seventh or eighth time that pundits have said 'McCain's campaign is in trouble,' " the senator said at an event in Mosinee, Wis., in a reference to his Lazarus-like resurgence to win his party's nomination. "We fooled them then, and we'll fool them again."

Policy matters took a back seat to character issues yesterday, but the campaigns did bicker over McCain's mortgage plan. The plan would spend as much as $300 billion to buy up troubled mortgages to stabilize the housing market. As initially announced Tuesday, the plan would have made lenders responsible for "the loss that they've already suffered." By Wednesday morning, the McCain campaign said that line was a mistake.

Obama pounced on the change yesterday as evidence that McCain favored banks over homeowners. "We have to act to fix our broken economy and restore the credit markets, but taxpayers shouldn't be asked to pick up the tab for the very folks who helped create this crisis," Obama said in Dayton.

For Obama, the back-and-forth over policy was less important than pressing the notion that McCain had moved unpredictably on an economic issue. He has been drumming the same theme for weeks.

"We need a steady hand in the White House. We need a president you can trust in times of crisis," Obama told a crowd in Cincinnati. He said McCain was "lurching all over the place" on the economy.

McCain, for his part, has used the Ayers issue sporadically in recent days, raising it in interviews but declining to do so in rallies or when on the same stage with Obama in Tuesday's debate. In making the connection more directly yesterday, McCain misstated the facts.

He described Ayers as someone "who still, at least on Sept. 11, 2001, said he still wanted to bomb more" — implying that was Ayers' response to the terrorist attacks of that day. McCain's reference was to a New York Times article coincidentally published Sept. 11, 2001 — based on an interview given before that date — which quoted Ayers as saying he did not regret setting bombs and that "we didn't do enough."

Ayers complained at the time that his quotes were juxtaposed and that he meant only that he felt he didn't do enough to stop the Vietnam War.