ST. PAUL, Sept 1 (Reuters) - The 17-year-old 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, the Palins said in a statement released by the campaign of Republican presidential candidate John McCain.

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

"We have been blessed with five wonderful children who we love with all our heart and mean everything to us," the Palins' statement said.

"Our beautiful daughter Bristol came to us with news that as parents we knew would make her grow up faster than we had ever planned. As Bristol faces the responsibilities of adulthood, she knows she has our unconditional love and support," the Palins said.

The Palins asked the news media to respect the young couple's privacy.

"Bristol and the young man she will marry are going to realize very quickly the difficulties of raising a child, which is why they will have the love and support of our entire family. We ask the media, respect our daughter and Levi's privacy as has always been the tradition of children of candidates," the statement concluded.

MCCAIN KNEW

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.

In the short period since she was announced last Friday, Palin has helped to energize the Republican Party's conservative base, giving the McCain camp fresh energy going into the campaign for the Nov. 4 election against Democrat Barack Obama.

McCain officials said the news of the daughter's pregnancy was being released to rebut what one aide called "mud-slinging and lies" circulating on liberal blog sites.

According to these rumors, Sarah Palin had faked a pregnancy and pretended to have given birth in May 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.

A senior McCain campaign official said the McCain camp was appalled that these rumors had not only been spread around liberal blog sites and partisan Democrats, but also were the subject of heightened interest from mainstream news media.

"The despicable rumors that have been spread by liberal blogs, some even with Barack Obama's name in them, is a real anchor around the Democratic ticket, pulling them down in the mud in a way that certainly juxtaposes themselves against their 'campaign of change,'" a senior aide said.