(CNN) Hillary Clinton stepped up her criticism of Democratic primary rival Bernie Sanders in a series of interviews Wednesday morning, calling Sanders "a pretty reliable vote for the gun lobby" and charging that his health care proposals "will cost middle-class families and working families."

Appearing on ABC's "Good Morning America" and NBC's "Today," Clinton was on the offensive against Sanders, targeting her upstart challenger after polls this week showed Sanders seizing leads in Iowa and New Hampshire, crucial early primary contests.

And as the Democratic race has tightened, open warfare has broken out between the Democratic opponents. Clinton has repeatedly hit the Vermont senator over his voting record on gun laws and opened a new line of attack on the cost of his health care policies -- arguments she continued Wednesday morning.

On "Today," the former Secretary of State said she wasn't nervous about Sanders' surge, and explained that "now it's time to draw some contrasts."

"One of the big ones, as you're aware, is on gun safety, where Senator Sanders has been a pretty reliable vote for the gun lobby," Clinton said. Sanders has in the past voted to support legal immunity for gun companies, which Clinton has used to batter her opponent.

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