SPRINGFIELD, Ill. — Scraping for any advantage in the presidential campaign’s waning days, John McCain and Barack Obama are introducing voters to a new cast of characters.

McCain would like people to know about a former 1960s radical and a corrupt government insider — both with links to his Democratic opponent.

And Obama is raising the Republican candidate’s connections to a disgraced savings and loan executive and a supporter of right-wing death squads.

Each candidate is trying to plant the idea that his opponent must be guilty of something if he has connections to such unsavory characters. Both candidates are guilty of stretching the facts, at times, to smear by association.

A closer look at the relationships being mentioned in campaign videos, ads and e-mails:

— William Ayers: Forty years ago, Ayers was a founder of the Weather Underground, a radical group that claimed responsibility for a series of bombings, including nonfatal explosions at the Pentagon and U.S. Capitol. He was a fugitive for years with his wife, fellow radical Bernadine Dohrn. But after Ayers surrendered in 1980, the charges against him were dropped because of prosecutorial misconduct.

Eventually, Ayers became a professor at the University of Illinois at Chicago, specializing in education reform and even advising Chicago Mayor Richard Daley.

In 1995, Ayers hosted a meet-the-candidate session at his home for Obama as he prepared to run for the state Senate. Later, the two worked with the same charity and social-service organizations in Chicago.

Obama has tried to minimize his link to Ayers, at one point saying he was just a guy who lived in the same neighborhood. But while there was more to the relationship than that, there is no evidence they were ever close friends or that Ayers advised Obama on policy. Obama has denounced Ayers’ violent activities, which occurred when Obama was a child.

— Charles Keating: Keating was a real estate speculator and savings and loan owner. His institution failed, costing many investors their life savings and sticking taxpayers with a $2.8 billion bailout cost.

McCain received $112,000 from him, his family and associates, and took trips to the Bahamas at Keating’s expense. McCain took up Keating’s cause with financial regulators who were investigating the businessman.

Keating eventually went to prison, and McCain received a mild rebuke from the Senate ethics committee. McCain repaid $112,000 to the U.S. Treasury and reimbursed Keating for the trips. He later said attending two meetings with bank regulators on behalf of Keating was “the worst mistake of my life” and that it had made him more sensitive to the need for government reform.

— Antoin “Tony” Rezko: Rezko was a key supporter and donor throughout Obama’s political career. Obama estimates Rezko raised $250,000 for his various campaigns, though not for his presidential bid. The two were friends who talked frequently about politics and occasionally dined out together with their wives.

Rezko was convicted this summer on federal charges of using his clout with state government to squeeze kickbacks out of firms wanting to do business with the state. The charges do not involve Obama. Rezko now appears to be cooperating with a federal probe of corruption in the Illinois government.

Obama consulted Rezko, a real estate developer, before buying a home in 2005. Rezko ended up buying a vacant lot next door and then selling some of the land to Obama. But Rezko did not help Obama pay for his house, and the sellers say Obama got no special treatment.