Bill Clinton Campaigning for Hillary in Iowa this Weekend

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DES MOINES, Iowa - While Hillary Clinton is locked in a tight race against Senator Bernie Sanders for Iowa Democrats' support on caucus night, she's pulling out her "secret weapon" on the campaign trail.

Former President Bill Clinton spoke to Iowa voters in Des Moines Saturday evening at Lincoln High School. So far, the former president has played a hands-off role in his wife's campaign for the Democratic nomination. But with three weeks until the Iowa caucuses, he's starting to speak up.

"She can't be president if you don't nominate. And if you do nominate her, I don't think they can stop her from becoming president," Clinton told a roaring crowd.

Clinton made a stop in Marshalltown Friday night before heading to Central Iowa Saturday. A central theme to Clinton's sales pitch for his wife involves her ability to find common ground with the GOP - something he suggests her competitors won't be able to do, should they win the White House.

"Everything she did, she tried to work with other people," the former president said.

Clinton was accompanied by his daughter, Chelsea Clinton, but Hillary was not present for this particular campaign tour.