UNDERWOOD, Iowa — Sen. Bernie Sanders, gunning for back-to-back wins in the first two presidential contests, stepped up his attacks on rival Hillary Clinton, comparing her to former Vice President Dick Cheney on foreign policy and suggesting she is in the pocket of a big Wall Street bank.

The charges came on a day of repeated barbs between the Clinton and Sanders camps and as a new poll in New Hampshire showed the Vermont senator’s lead widening in the nation’s first primary contest.