Richard Wolf

USA TODAY

Political advice from Bernie Sanders to Donald Trump: Give Bill Clinton's sex life a rest.

The Vermont senator, who's chasing Hillary Clinton in the Democratic presidential primaries, said Sunday that Trump's focus on the former president's affair with Monica Lewinsky will backfire among women voters. Bill Clinton is set to hit the campaign trail on behalf of his wife this month.

"We have enormous problems facing this country, and I think we got more things to worry about than Bill Clinton’s sexual life," Sanders said on ABC's "This Week," citing climate change, raising the minimum wage and doing away with tax breaks for billionaires as examples. "Maybe he should focus on those things."

Trump has raised Bill Clinton's affair and impeachment in 1998 as a way of responding to Hillary Clinton's popularity among women, which he attributes to her "playing the woman card." Among presidential candidates, "I'm the only one that's willing to talk about his problems," Trump said on CBS' "Face the Nation" Sunday.

While Trump was focusing his attacks on the Clintons, Sanders called Trump a "pathological liar" -- for instance, by claiming that thousands of Muslims in New Jersey celebrated the Sept. 11 attacks as the World Trade Center was coming down. "Time after time, this guy just comes up with things off the top of his head that are lies," Sanders said. "And somebody has got to say that he is a pathological liar."

Sanders also backed President Obama on gun control, despite his own past support for Second Amendment rights as a senator from a largely rural state. Obama is considering an executive action on guns that would go around Congress.

"There is a wide consensus, overwhelming majority of the American people believe we should expand and strengthen the instant background check" to keep guns out of the hands of criminals and people with mental health issues, he said. "I think that’s what the president is trying to do, and I think that will be the right thing to do."