Johnston also wrote: "I don't want kids."

Some US commentators have held up the example of Bristol's teen pregnancy to say Mr McCain had not vetted Mrs Palin and her family circumstances properly before appointing her as his running mate.

Mr McCain reportedly met Mrs Palin only once before offering her the job as vice-presidential nominee.

"A series of disclosures about Governor Sarah Palin, Senator John McCain's choice as running mate, called into question on Monday how thoroughly Mr McCain had examined her background before putting her on the Republican presidential ticket," The New York Times reported.

Much has been written about Mrs Palin's links to the Alaskan Independence Party, a group pushing for a vote on Alaska splitting from the rest of the US and forming an independent nation.

The New York Times said that Mrs Palin "was a member for two years in the 1990s". However, The Associated Press reported that, while Mrs Palin's husband, Todd, had been registered as a member, she had only addressed the party.

Even US actress Lindsay Lohan, often the focus of the media herself, has weighed in about Mrs Palin.

"I've been watching the news all morning, like everyone else - and keep hearing about the issues related to 'teen pregnancy'," Lohan wrote on her blog.

"Well, I think the real problem comes from the fact that we are taking the focus off of getting to know Sarah Palin and her political views, and what she can do to make our country a less destructive place."

Mrs Palin's emergence has also focused attention on Hillary Clinton, who made a historic bid to become the first female presidential nominee before succumbing to Barack Obama.

Senator Clinton, who is now barracking for Senator Obama, has been relatively quiet about Mrs Palin's appointment, other than to say everybody "should all be proud of Governor Sarah Palin's historic nomination".

Ironically, McCain supporters have credited Senator Clinton's efforts with paving the way for Mrs Palin.

"Because of Hillary Clinton's historic run for the presidency and the treatment she received, American women are more highly tuned than ever to recognise and decry sexism in all its forms," said top McCain campaign adviser Carly Fiorina.

But Senator Clinton was "galled" that Mrs Palin might try to capitalise on the impact she made among women voters during the Democratic primaries, The New York Times reported.

The paper predicted Senator Clinton would now play a greater role campaigning for Senator Obama as a result.

Conservative columnists have decried the treatment of Mrs Palin in the press. Conservative blogger Michelle Malkin labelled "attacks" on Mrs Palin as "Palin Derangement Syndrome", and said there was "selective sympathy" for mothers who ran for office.