Is Sarah Palin becoming a liability for John McCain?

United States - Only the most avid followers of American presidential politics - or those of a certain age - will remember Thomas Eagleton.

The Missouri senator was George McGovern's vice-presidential candidate for 18 days in 1972. He resigned when it was revealed he'd had electro-shock treatments to deal with his mental health issues - something he'd deliberately kept from his Democratic running mate.

Eagleton was replaced by Sargent Shriver, husband of Eunice Kennedy, and the McGovern-Shriver ticket went on to be roundly defeated by Richard Nixon and Spiro Agnew.

Some have already drawn parallels between Eagleton and Sarah Palin, John McCain's controversial choice as running mate. The Huffington Post's Richard Gizbert referred to her as "Thomas Eagleton: The Sequel" when it was revealed her teenaged daughter was pregnant.

While Palin has no known history of severe mental illness, even the Alaska governor's usual cheerleaders on the right have been turning on her in recent days, including one conservative commentator who urged her to drop out of the race in the wake of a series of cringe-worthy media interviews.

"As we've seen and heard more from John McCain's running mate, it is increasingly clear that Palin is a problem," Kathleen Parker, who once described her candidacy as a "bright light," wrote on the National Review Online.

"Quick study or not, she doesn't know enough about economics and foreign policy to make Americans comfortable with a President Palin should conditions warrant her promotion."

Parker pleaded with Palin to resign by citing family obligations if necessary, telling her: "Do it for your country."

Amid this backdrop, the vice-presidential debate will be held in St. Louis this Thursday, and Republican strategists are said to be beside themselves in their attempts to prepare Palin for the faceoff against Senator Joe Biden.

Ed Schultz, a liberal syndicated talk-radio host, wrote on his blog on the weekend that his sources on Capitol Hill have told him the McCain campaign had held "disastrous" mock debates and a mock news conference with Palin.

"One senior McCain aide was quoted as saying, 'What are we going to do?' The McCain people want to move this first debate to some later, undetermined date, possibly never. People on the inside are saying the Alaska governor is clueless," Schultz wrote.

McCain is continuing to sag in the polls following his much-maligned interference in the bipartisan negotiations in Washington into a proposed $700 billion bailout of the country's failing banks and financial institutions. The deal was finally hammered out early Sunday.

His failure to land any knockout punches against Barack Obama during their own debate Friday night in Mississippi isn't likely to improve his standing in the polls in the days to come.

So could a flailing McCain, seemingly hungry for the White House and known for high-stakes political gambles - including his choice of Palin in the first place - have another surprise up his sleeve by cutting the Alaska governor loose with less than 40 days til the election?

"They've got a real problem," Andrea Mitchell, NBC's chief foreign affairs correspondent, told "The Chris Matthews Show" on Sunday morning.

"She's a big draw in terms of attracting crowds and money ...but if she continues this way, she could cost them this election."

But Stephen Hess, a senior fellow at the Brookings Institute think-tank, doesn't believe that even a daring politician like McCain can dump his running mate now, regardless of how deftly his campaign might make it appear that it's Palin's idea to bail.

"It's not going to happen," said Hess, who worked for the Dwight Eisenhower and Nixon campaigns.

"Presidents, or those who choose to be president, don't like to admit their mistakes, in part because others would take advantage of that. Eagleton was a special case because he hadn't told McGovern anything about his mental health or that he'd had shock treatment, so it was a great embarrassment."

In 1968, Hess was in charge of handling another unpopular vice-presidential candidate - Agnew, the governor of Maryland who was later forced out of the vice president's office when he was charged with accepting bribes and tax evasion.

Hess says he sees more parallels between McCain's problems with Palin and Nixon's issues with Agnew.

"Spiro Agnew was a governor of a relatively small state, and he was someone the candidate did not know, or knew barely, and he chose him for purely political reasons - he wanted someone viewed as more moderate," Hess recalls.

Soon after Nixon chose his running mate, Hess said, Agnew made a number of statements deeply embarrassing to the campaign, the most infamous being his remark when he travelled to Detroit: "You've seen one city slum, you've seen them all."

But Nixon stuck with him, Hess noted.

"Even fully knowing Agnew's problems, except that he was a crook, he ran with him a second term. That's how far presidents will go not to admit their mistakes."