WATCH: Hillary Clinton accuses Bernie Sanders of being a tool of the NRA

U.S. Democratic presidential front-runner Hillary Clinton accused top challenger Bernie Sanders of being too lax on gun control at a debate on Sunday in a bid to block Sanders' recent rise in opinion polls that threaten her in the early voting states of Iowa and New Hampshire.

At a debate in Charleston, South Carolina, not far from a historic African-American church where nine people were shot to death last summer, Clinton pounced on Sanders in their last face-to-face encounter until Iowa kicks off the presidential nominating race in two weeks.

Clinton welcomed Sanders' decision on Saturday night to back a bill in Congress rescinding portions of a law giving immunity from lawsuits to gunmakers.

But the former secretary of state said the U.S. senator's past record showed a more lenient attitude toward the demands of the National Rifle Association (NRA) gun lobby.

"He voted to let guns go on Amtrak (trains), guns go into national parks. He voted against doing research to figure out how we can save lives. Let's not forget what this is about: 90 people a day die of gun violence in our country," Clinton said.

Sanders, a self-styled democratic socialist from Vermont, defended himself, saying he has a strong record on trying to prevent guns from getting into the wrong hands and standing up against the powerful NRA.

"I think Secretary Clinton knows what she says is very disingenuous," said Sanders.

He also said he believed he would be able to win over African-American voters, noting that when his presidential campaign began, Clinton was 50 percentage points ahead of him in the polls.

"Guess what: In Iowa, New Hampshire, the race is very, very close," he said.

The leading Democratic contenders have stepped up their attacks on each other during the past week, battling over guns, healthcare and Wall Street with growing intensity as polls showed Sanders gaining ground on Clinton in key states.

Clinton and Sanders were joined by former Maryland Governor Martin O'Malley, who lags badly in polls, in what was the fourth debate between the Democratic contenders.

Sanders has pulled into a statistical tie with Clinton in recent polls in Iowa, which holds the first contest on Feb. 1 in the race to pick a nominee for the November election. He also leads Clinton in the next state to vote, New Hampshire, on Feb. 9, according to polls.

Hours before the debate, Sanders answered Clinton's demand to explain funding for his healthcare plan, proposing a "Medicare-for-all" system funded by a 2.2 percent "premium" on individuals and a 6.2 percent payroll tax paid by employers.

The plan also includes a new estate tax on the wealthiest Americans and changes in the tax code to make rates more progressive. The top rate, 52 percent, would apply to those making more than $10 million a year.

"Senator Sanders has been changing a lot of positions in the last 24 hours because when his plans and record come under scrutiny, their very real flaws get exposed," Clinton said in a statement responding to the plan.

(Writing by Steve Holland and John Whitesides; Editing by David Gregorio, Mary Milliken and Jonathan Oatis)