Republican presidential frontrunner Donald Trump, increasingly focusing on Hillary Clinton rather than his GOP rivals, says he's not concerned by reports Bill Clinton will soon hit the trail for the Clinton campaign.

"I think that Bill Clinton turned out to be a liability against Obama when [Hillary Clinton and Barack Obama] ran against each other in 2008," Trump said Sunday morning on "Fox and Friends." "If you look at that whole campaign, it was a mess. [Bill Clinton] said things that were very problematic and troublesome…certainly Bill Clinton is not going to be the end-all in the campaign."

Trump also explained his charge, made in a tweet Saturday night, that Bill Clinton has a "penchant for sexism." In the tweet, Trump said "Hillary Clinton has announced that she is letting her husband out to campaign but HE'S DEMONSTRATED A PENCHANT FOR SEXISM, so inappropriate!"

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