(CNN) -- Sen. Barack Obama on Thursday slammed Sen. John McCain's new mortgage plan as "the latest in a series of shifting positions" and evidence of "erratic and uncertain leadership."

"He's ended up with a plan that punishes taxpayers, rewards banks, and won't solve our housing crisis," Obama said at an event in Dayton, Ohio.

At the second presidential debate Tuesday, McCain suggested that the government buy up bad home loan mortgages and renegotiate at the new diminished value of those homes.

Under McCain's proposed $300 billion mortgage rescue plan, much of the burden of paying to keep troubled borrowers in their homes would shift to taxpayers.

McCain's original plan called for lenders to write down the value of these mortgages and take those losses.

Doug Holtz-Eakin, McCain's economic adviser, said Wednesday that the McCain plan could be put into place quickly because the groundwork and the authority for it already have been provided by last week's $700 billion bailout bill; the Hope for Homeowners program authorized by the housing rescue bill passed in July; and the government takeover of mortgage giants Fannie Mae and Freddie Mac. Read the details of McCain's proposal

In an e-mail to supporters, McCain said his plan would address "the very root of the failing housing market."

Obama charged that McCain's latest plan is "risky."

"I don't think we can afford that kind of erratic and uncertain leadership in these uncertain times. We need steady leadership in the White House. We need a president we can trust in times of crisis," Obama said.

In response to Obama's remarks in Dayton, the McCain campaign said Obama was "putting politics above the national interest."

"John McCain's home ownership resurgence plan represents absolutely no new expense to the taxpayer, but simply refocuses priorities to more directly assist the homeowners who are hurting instead of greed on Wall Street," spokesman Tucker Bounds said in a statement.

Earlier Wednesday, Obama's campaign launched a television ad that takes aim at McCain's proposal, saying "the same lenders that caused the crisis in the first place" would benefit from the plan.

As the candidates enter the final weeks of campaigning, they have been attacking each other more aggressively.

McCain's campaign has launched a string of new ads that question Obama's judgment and character.

The McCain campaign calls Obama "too risky for America" in a new Web ad that focuses on his political relationship with Bill Ayers, a founding member of the radical Weather Underground.

"Barack Obama and domestic terrorist Bill Ayers. Friends. They've worked together for years. But Obama tries to hide it," the announcer says in the 90-second ad.

"But Obama's friendship with terrorist Ayers isn't the issue. The issue is Barack Obama's judgment and candor," the announcer says. Watch analysts weigh in on the McCain attacks »

At a town hall meeting in Waukesha, Wisconsin, angry voters pleaded with McCain to get tougher on Obama.

One voter suggested that McCain bring up the Rev. Jeremiah Wright controversy.

"I am begging you, sir, take it to him," the voter said.

McCain did not specifically address the comment about Wright, Obama's former pastor who came under scrutiny during the primaries after clips of his sermons circulated on the Internet.

On Ayers, the Arizona senator said he doesn't care about a "washed-up former terrorist," but vowed to press Obama on his candor and judgment.

"We need to know the full extent of the relationship because of whether Sen. Obama is telling the truth to the American people or not. That's the question," McCain said.

Weather Underground was involved in bombings in the early 1970s, including attacks on the Pentagon and the Capitol. Obama was a young child at the time of the bombings.

Obama and Ayers, now a university professor, met in 1995, when both worked with a nonprofit group trying to raise funds for a school improvement project and a charitable foundation. CNN's review of project records found nothing to suggest anything inappropriate in the volunteer projects in which the two men were involved. Fact check: How close are Obama and Ayers?

Quoted in The New York Times, Obama called Ayers "somebody who engaged in detestable acts 40 years ago, when I was 8."

Michelle Obama brushed off the latest round of attacks in an interview with CNN's Larry King on Wednesday.

"I don't know anyone in Chicago who's heavily involved in education policy who doesn't know Bill Ayers. But, again, I go back to the point that the American people aren't asking these questions," she said. Watch Michelle Obama's interview »

Obama said her husband has been "thoroughly vetted" and said the Ayers allegations are a "part of politics."

The McCain campaign started pushing hard on the Ayers connection this past weekend when Alaska Gov. Sarah Palin accused Obama of "palling around with terrorists who would target their own country."

The Obama campaign retaliated Monday with a 13-minute documentary Web video detailing the Arizona senator's involvement in the Keating Five scandal in the 1980s.

Cindy McCain, McCain's wife, lashed out at Obama earlier this week, telling a Tennessee newspaper that the Illinois senator has waged the "dirtiest campaign in American history."

Obama's running mate, Sen. Joe Biden put the blame on the McCain campaign Wednesday, saying the attacks on Obama are "beyond disappointing."

CNNMoney.com's Les Christie contributed to this report.

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