In another broadside of personal attacks against Democratic Sen. Barack Obama, Sen. John McCain released a TV ad on Friday seeking to link his rival for the presidency with a 1960s radical.

"When convenient, he worked with terrorist Bill Ayers. When discovered, he lied" because he has "blind ambition" and "bad judgment," the ad says.

Obama on Friday said McCain is trying to divide the country with angry remarks and TV ads.

Obama's campaign also announced Friday that it is buying a half-hour block of prime-time TV a few days before Election Day. (Read Story)

"It's not hard to rile up a crowd by stoking anger and division," Obama told Ohio voters.

"We’re in a serious crisis -- now, more than ever, it is time to put country ahead of politics," he said. (Video: McCain Crowds Get Angry)

The ad comes in the same week that McCain and his vice presidential running mate, Sarah Palin, have suggested that Obama is hiding links to a man they call a "domestic terrorist," Palin has sought to reinvigorate a debate over Obama's controversial former minister, a McCain supporter hurled a racial epithet at a black man in the press pool and demanded that he "sit down, boy," and Palin supporters shouted "treason" as she talked about the Democrat. On Thursday, former Oklahoma Gov. Frank Keating, a co-chairman of Republican John McCain's presidential campaign, called Obama a "guy of the street" in a radio interview and said the Democratic hopeful should be more candid about his youthful drug use.

In his memoir "Dreams From My Father," first published in 1995 and re-released in 2004, Obama described experimenting with alcohol and drugs as a teenager, including marijuana and cocaine when he could afford it.

"Junkie. Pothead. That's where I'd been headed: the final, fatal role of the young would-be black man," Obama wrote.

Ayers, who teaches at the University of Illinois, is a former leading radical in the 1960s and helped to found the violent Weather Underground group, whose members were blamed for several bombings when Barack Obama was 8 years old. His father, Thomas G. Ayers, was once CEO of Commonwealth Edison, the local power company.

Facing federal charges, he went underground with other members of the group including Bernardine Dohrn, whom he married.

Since emerging, Ayers has earned a bachelor's, master's and doctorate degree in education and is Distinguished Professor at the University of Illinois, Chicago, College of Education. He has published a wide range of scholarly works, and serves on the board of the philanthropic Woods Foundation.

"He’s done a lot of good in this city and nationally," Chicago Mayor Richard M. Daley said in an interview with the New York Times recently.

It was through his philanthropic work that Ayers first met Obama.

Ayers helped author a Chicago Annenberg Challenge grant proposal that in 1995 won $49.2 million over five years for public school reform. Obama was one of six people tapped to oversee the distribution of grants. They attended six board meetings together early in the project.

Later in 1995, Ayers and Dohrn hosted a gathering in their Hyde Park neighborhood for local Democrats to meet Obama, after state Sen. Alice J. Palmer wanted Obama to run for her seat as she ran for Congress.

The New York Times reported that Obama and Ayers crossed paths again in 2000-2002 because they both served on the board of the Woods Foundation; the philanthropy had supported Obama's work as a community organizer. Ayers once made a campaign donation to Obama of $200 when he served in the Illinois State House.

Obama has repeatedly denounced Ayers' past actions.

The tone of the Republicans' campaign has taken to sharp personal attacks as national and battleground state polls show the McCain-Palin ticket losing ground, and Americans increasingly concerned about the nation's financial problems. (Interactive: Poll Tracker)

"It's a dangerous road, but we have no choice," a top McCain strategist told the New York Daily News last week. "If we keep talking about the economic crisis, we're going to lose."

The Illinois senator said American's aren't looking for someone who can divide the country, but "they are looking for someone who can lead this country."

"Now more than ever it is time to put country ahead of politics," he said. Obama is criticizing McCain's economic plans during his two-day Ohio tour.