As Bernie Sanders exudes confidence in his ability to win next week’s Iowa caucuses, his moderate rivals are struggling with how — and whether — to directly take on the progressive Vermont senator who some Democrats worry won’t be able to defeat President Donald Trump. Former Vice President Joe Biden jabbed Sanders — without naming him — on the multitrillion-dollar cost of his most ambitious proposals, such as single-payer health insurance, tuition-free college and sweeping climate action. “I don’t think you win votes doing that,” Biden said Monday in Cedar Falls.