MONROE -- Barack Obama said Monday he was offended by claims his campaign fanned rumors about Republican vice presidential pick Sarah Palin, telling the press to "back off" stories about her pregnant daughter.

The Democratic White House nominee was speaking following the revelation that Palin's 17-year-old unmarried daughter Bristol was expecting and would keep the baby and marry its father.

Earlier, an unnamed McCain campaign aide was quoted as condemning liberal blogs for spreading rumors that Bristol was the mother of the infant the Alaska governor recently gave birth to, and said some postings contained Obama's name.

"I am offended by that statement, there is no evidence at all that any of this involved us," Obama told reporters.

"I hope I am as clear as I can be. So in case I am not, let me repeat, we don't go after people's families, we don't get them involved in politics. It is not appropriate and it is not relevant," Obama told reporters in Michigan.

"Our people were not involved in any way in this and they will not be, and if I ever thought it was somebody in the campaign that was involved in something like that they would be fired."

Obama also noted that his late mother had given birth to him when she was only 18.

"I would strongly urge people to back off these kinds of stories. How a family deals with issues and teenage children that shouldn't be the topic of our politics and I hope that anybody who is supporting me understands that is off limits."