ST. PAUL (Reuters) - The 17-year-old unmarried daughter of Republican vice presidential candidate Sarah Palin is pregnant, Palin said on Monday in an announcement intended to knock down rumors by liberal bloggers that Palin faked her own pregnancy to cover up for her child.

Bristol Palin, one of Alaska Gov. Palin’s five children with her husband Todd, is about five months pregnant and is going to keep the child and marry the father, according to aides of Republican presidential candidate John McCain.

Bristol Palin made the decision on her own to keep the baby, the aides said.

The Palins, in a statement released by the McCain campaign, said Bristol “came to us with news that we as parents knew would make her grow up faster than we had ever planned” and that their daughter “has our unconditional love and support.”

“We ask the media to respect our daughter and Levi’s privacy as has always been the tradition of children of candidates,” their statement said.

Senior McCain campaign officials said McCain knew of the daughter’s pregnancy when he selected Palin last week as his vice presidential running mate, deciding that it did not disqualify the 44-year-old governor in any way.

McCain officials said the news of the daughter’s pregnancy was being released to rebut what one aide called “mud-slinging and lies” that have circulated on liberal blog sites.

According to these rumors, Sarah Palin had faked a pregnancy and pretended to have given birth in April to her fifth child, a son named Trig who has Down syndrome. The rumor was that Trig was actually Bristol Palin’s child and that Sarah Palin was the grandmother.

PRO-LIFE GROUPS OFFER SUPPORT

Palin is staunchly anti-abortion, and pro-life groups welcomed the decision to keep the child.

James Dobson, an influential Christian evangelical conservative, said his Focus on the Family group had always counseled young mothers to see their pregnancies through, “even though there will be of course challenges along the way.”

“That is what the Palins are doing, and they should be commended once again for not just talking about their pro-life and pro-family values, but living them out even in the midst of trying circumstances,” he said in a statement.

Charmaine Yoest, head of Americans United for Life, said, “We join them in welcoming this new life.”

Slideshow ( 5 images )

The McCain campaign was outraged by the blog rumors.

“There’s no doubt that liberal blogs such as one called www.barackoblogger.com and some in the mainstream media were pushing a false story about Gov. Palin’s most recent pregnancy with fervor,” said senior McCain adviser Nicolle Wallace.

A senior McCain official said its camp had no evidence that the campaign of Democrat Barack Obama was pushing the story, but said the blog rumors circulating on websites that appeared to support Obama had the effect of being “a real anchor around the Democratic ticket.”

Slideshow ( 5 images )

OBAMA OFFENDED

Speaking to reporters in Monroe, Michigan, Obama said he was offended by the McCain aide’s statement and that he considered people’s families off-limits.

“We don’t go after people’s families. We don’t get them involved in the politics. It’s not appropriate and it’s not relevant. Our people were not involved in any way in this and they will not be. And if I ever thought there was somebody in my campaign that was involved in something like that, they’d be fired,” Obama said.

Obama also said: “This shouldn’t be part of our politics. It has no relevance to Gov. Palin’s performance as a governor or potential performance as a vice president. So I would strongly urge people to back off these kinds of stories.”

The news broke as Republicans gathered in St. Paul for their convention to formally nominate McCain and Palin as the party’s candidates. Palin is only the second woman picked as a U.S. vice presidential nominee.

In the short period since she was announced last Friday, Palin has helped to energize the Republican Party’s conservative base, giving McCain fresh energy going into the campaign for the November 4 election against Obama and his No. 2 Sen. Joe Biden of Delaware.

(Additional reporting by Caren Bohan and Ed Stoddard)