U.S. Democratic presidential front-runner Hillary Clinton went on the offensive against top challenger Bernie Sanders at a debate on Sunday in reaction to his recent rise in opinion polls, saying he is soft on gun control and that his proposed health care overhaul is unrealistic.

Clinton, who leads in polls nationally but has seen Sanders gain in the early-voting states of Iowa and New Hampshire, tried to raise questions repeatedly about the self-styled democratic socialist at their last face-to-face encounter before Iowa holds the first nominating contest of 2016 on Feb. 1.

Clinton pounced on Sanders' "Medicare-for-all" plan that was announced just hours before the debate and came in response to Clinton's criticism of his previous record on health care over his career as a U.S. senator from Vermont.