Tightening Clinton-Sanders battle raises stakes for Democratic debate
Democratic presidential candidate Hillary Clinton speaks to 'Hardball' host Chris Matthews on Jan. 5, 2016. (MSNBC)

Tightening polls and rising tensions between White House rivals Hillary Clinton and Bernie Sanders could fuel a fiery Democratic debate on Sunday, their last face-to-face encounter until Iowa kicks off the presidential nominating race in two weeks.

The leading Democratic contenders 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, Sanders and former Maryland Governor Martin O'Malley, who lags badly in polls, will participate in the 9 p.m. EST (0200 GMT on Monday) debate, the fourth between the Democratic contenders.

Foreign policy also could play a role in the debate, which follows Saturday's prisoner deal announced by the United States and Iran. Tehran has freed five Americans including a Washington Post reporter and a Christian minister, coinciding with the lifting of most international nuclear sanctions on Iran.

Republican candidates expressed relief at the prisoner release but renewed their criticism of President Barack Obama's willingness to make deals with Tehran, particularly his earlier agreement with Iran to curtail its nuclear ambitions. Sanders and Clinton have both supported the Iran nuclear deal.

Sanders, a U.S. senator from Vermont, 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.

As the race has tightened, Clinton has been on the attack. The former secretary of state and U.S. senator from New York has hammered Sanders for past votes to support immunity from lawsuits for gun manufacturers and criticized his call for a national single-payer healthcare system. She demanded details on how he would pay for it.

Sanders fired back with an ad criticizing Democrats who take money from Wall Street, an obvious dig at Clinton, and touted his plan to break up the big banks. An angry Clinton campaign quickly accused Sanders of breaking his pledge not to air negative ads against her.

"We have differences and that's what I'm focusing on now. We're going to have a spirited debate, I expect, tonight in Charleston," Clinton said on ABC's "This Week" on Sunday.

Clinton stepped up her attacks on Sanders as too soft on gun restrictions ahead of the debate, which will be held in Charleston, South Carolina, one block from the historic church where nine black worshipers were killed by a white gunman in June.

She welcomed his decision on Saturday night to back a bill in Congress rescinding portions of a law giving immunity from lawsuits to the gunmakers, but said he had "flip-flopped" and should also back a proposal to extend the three-day waiting period on background checks.

Sanders, who attended Sunday services at the Charleston church where the murders occurred, said he would consider supporting a longer waiting period. He called the Clinton offensive a sign of her growing anxiety.

"I think the reason that the Clinton campaign is getting defensive is they see that we have the momentum," Sanders said on NBC's "Meet the Press."

Wins by Sanders in both Iowa and New Hampshire would be a huge blow to Clinton, long seen as a prohibitive favorite. After those two states, the race moves on to Nevada and South Carolina, where Clinton leads in polls, and a March 1 round of 11 state contests.

"Things could change radically here if Bernie wins in Iowa and New Hampshire," said Phil Noble, a veteran South Carolina party strategist and head of the state's New Democrats, who has endorsed O'Malley.

Brad Anderson, an Iowa-based Democratic strategist who was state director for President Barack Obama's 2012 campaign, said the debate could play a part in deciding the outcome in Iowa.

"The debate is going to be enormously important given all the undecideds we are seeing in polls," said Anderson, who supports Clinton. "People are still really, really weighing their decision here."

(Editing by David Gregorio and Mary Milliken)