Democratic presidential candidate Bernie Sanders,., holds up a fist as he arrives to speak at a veterans forum in Marshalltown, Iowa, Sunday. (Andrew Harnik/AP)

The two leading candidates for the Democratic presidential nomination intensified their attacks on each other Sunday, reflecting a tightening race in the early-contest states.

Hillary Clinton and Bernie Sanders are each taking aim at the other’s political strengths, not their weaknesses. The former secretary of state and presumptive Democratic favorite is emphasizing the issue of gun control as a testament to her liberalism — and trying to undermine Sanders’s case that he is more in tune with the passions of the party’s left-leaning base.

Meanwhile, the senator from Vermont, a self-described “democratic socialist,” is making the argument that he is the more electable candidate against the ultimate Republican nominee. Clinton’s claim to electability has been central to her appeal to Democrats’ pragmatism.

Fresh evidence of how close things stand in Iowa and New Hampshire came Sunday with the release of a poll by the Wall Street Journal, NBC News and Marist College. It showed Clinton and Sanders running statistically even among likely voters in the two states.

Yet the dynamics in Iowa and New Hampshire — both set to vote in less than a month — appear to be shifting in opposite directions. In Iowa, Clinton’s once-healthy lead seems to be eroding, while in New Hampshire, it is Sanders who is seeing his advantage slip away.

Democratic presidential candidate Hillary Clinton addresses an audience during an event Sunday in Hooksett, N.H., at which Planned Parenthood endorsed her. (Steven Senne/AP)

[Hillary Clinton could win Iowa but still lose the battle of expectations]

More telling than any single poll is the jujitsu-like manner in which Clinton and Sanders are battling.

Clinton, who in their last debate all but ignored her Democratic opponent to focus on GOP front-runner Donald Trump, has begun pounding Sanders for past votes that reflected the sentiments of his rural Vermont constituents in favor of gun rights.

Sanders’s 2005 vote in support of legal protections for gun manufacturers is a “difference Democratic voters in our primary can take note of,” Clinton told CBS. She pointed out that she and President Obama, then both senators, voted the other way.

It was the second television interview in three days in which Clinton hammered Sanders directly and by name about gun control, an issue of renewed significance to Democrats this cycle.

[Sanders tells Iowa crowd he supports changes to gun manufacturers liability law]

Sanders, at a campaign stop in central Iowa, suggested the topic was getting a lot of attention because Clinton sees him gaining on her.

Members of the audience applaud as Democratic presidential candidate Sen. Bernie Sanders speaks at a veterans forum in Marshalltown, Iowa, Sunday. (Andrew Harnik/AP)

“The polls are now showing that we have significant momentum, and we are only a few points behind Secretary Clinton,” he told a crowd of about 600 people in Marshalltown. “Suddenly the candidate who was ordained by the establishment to win, was the undisputed leader, suddenly finds that her position is not quite so strong. You’re going to see a lot of political discussion and a lot of attacks taking place.”

Meanwhile, the Sanders campaign was touting the Journal-NBC-Marist poll’s surprising numbers indicating that he would do better than Clinton in both Iowa and New Hampshire against the leading Republican contenders: billionaire Donald Trump and senators Ted Cruz (Tex.) and Marco Rubio (Fla.).

The reason, said Marist polling director Lee Miringoff, is that Sanders polled better than Clinton among independent voters.

Clinton issued a rebuttal Sunday with a television ad in Iowa and New Hampshire that does not name Sanders but argues implicitly that he cannot defeat Republicans in the fall. It includes clips of Trump, Cruz and other Republicans talking about war, abortion, health coverage, wages and other subjects. “Think about it. One of these Republicans could actually be president,” an announcer says. “So ask yourself, who is the one candidate who can stop them? Hillary Clinton. Tested and tough. To stop them, stand with her.”

[Bernie Sanders says he’d ‘love, love, love’ to run against Donald Trump this fall]

The campaign also sent a Twitter message featuring a young New Hampshire voter who switched allegiance from Sanders to Clinton.

“This 19-year-old voter in New Hampshire was on the fence until he heard @billclinton speak on Hillary’s experience,” the Clinton campaign said. Former president Bill Clinton made his first campaign appearances for his wife last week.

The tweet linked to a story about Alex Mendola of Amherst, a town in southern New Hampshire.

“I was a solid Bernie voter, but now I’m not so sure,” Mendola was quoted as saying. “If Bernie won the primary and lost the general election, I think that would be disaster. So even if [I] don’t like Hillary as much as Bernie, I feel more confident that she would win the general election.”

Beyond the polling and the scrapping by the two candidates themselves, the fluidity of the race can be heard in the voices of individual voters.

Les Davis, a retired carpenter who came to see Sanders in Marshalltown, said he had been leaning in Clinton’s direction until recently.

“I probably agree with the majority of her issues, and I don’t want to sound sexist here, but I think having a woman would probably be a good thing,” said Davis, 67.

But he said he’s been more inspired by Sanders, particularly his talk about taking on Wall Street and breaking up the banks. Davis also believes his daughter and son-in-law could have a brighter future under Sanders’s economic policies.

And his doubts about Sanders’s electability are waning.

“Obama came from behind, he was kind of unknown, and I’d like to see Bernie do the same,” Davis said.

Indeed, a defeat in Iowa on Feb. 1 would be a reminder of how Clinton’s once-formidable candidacy crumpled in this state in 2008 against a first-term senator from Illinois.

Clinton’s lead had been steady, if not wholly comfortable, at more than five percentage points for most of the fall. Lately, her campaign has been quietly lowering expectations among key supporters and the media for a blowout victory in Iowa.

“From the beginning, we’ve expected and prepared for a competitive caucus by working hard and building a durable organization,” said Matt Paul, who is leading Clinton’s Iowa operation. “Hillary Clinton, our volunteers, and our organizers will continue to campaign hard across the state to earn every vote.”

Most polling lapsed in late December because of the holidays, which meant that the numbers in the survey released Sunday were the first snapshot of the race lately.

“You were once way ahead. So what’s happening?” John Dickerson, host of CBS’s “Face the Nation,” asked Clinton on Sunday.

“Oh, John, you know, these polls go up, they go down,” Clinton replied. “I stay pretty focused, as I think we all should, on what we have to do to build on the progress of the Obama administration but go even further.”

Sanders’s advisers believe they are seeing the payoff in Iowa of both their ground efforts — including the consistently large crowds the senator is drawing — and their television advertising, which got off to a later start than Clinton’s.

Since May, 40,000 people have attended Sanders campaign events, according to a tally kept by aides. By the caucuses, the Sanders camp believes he will have spoken in person to more than 50,000 people.

“That’s a huge percentage of people who are actually going to participate in the caucuses,” said Jeff Weaver, Sanders’s campaign manager, who traveled with the candidate in Iowa over the weekend. “There’s nothing more compelling than voters having an opportunity to hear Senator Sanders in person and see other people moved by his message as well.”

Wagner reported from Marshalltown and Gearan from Washington. Dan Balz and Philip Rucker in Des Moines contributed to this report.