Hillary Clinton is sharpening her attacks on Bernie Sanders – and even her criticism of the Obama administration – as polls show her losing ground to her closest primary rival in key states.

Three weeks before the leadoff Iowa caucuses, Clinton appears to be changing up her strategy of looking past the Vermont senator, instead going after his record on gun control, taxes, health care and even the Wall Street regulations that have been the core of his insurgent campaign.

Clinton, picking up the endorsement of the Brady Campaign to Prevent Gun Violence, cast Sanders Tuesday as weak on gun control.  

"If you're going to go around saying you stand up to special interests then stand up to that most powerful special interest -- stand up to the gun lobby," she said.

That was after claiming Monday that Sanders would "rip up" ObamaCare and put power in the hands of states. She also came out against the Obama administration’s raids targeting Central American immigrants who entered the U.S. illegally and ignored deportation orders.

The toughened tone reflects fresh turbulence in the race for the Democratic front-runner, as she struggles to consolidate support among the various wings of the Democratic Party. While she’s locked down endorsements from major groups like Planned Parenthood, Sanders won the backing Tuesday of the liberal MoveOn.org.

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Even Vice President Biden, who flirted with running before deciding against it last year, took an implicit swipe at Clinton by suggesting she’s late to the cause on the issue of income inequality.

"Hillary's focus has been other things up to now, and that's been Bernie's -- no one questions Bernie's authenticity on those issues," Biden said in a CNN interview.

In the most recent Fox News poll, Sanders expanded his lead over Clinton in New Hampshire, building a 13-point advantage over the former secretary of state. New Hampshire is friendly territory for the senator from the neighboring state, but even in Iowa, polls have started to tighten.

Speaking with Fox News, Clinton campaign chairman John Podesta acknowledged he’s hearing some concerns among fellow Democrats about Sanders gaining ground in the two early-voting states.

Podesta told Fox News that in Iowa, specifically, the race is “tightening.” But Podesta stressed the Clinton camp still feels the “fundamentals” of their campaign, including the ground game, are strong.

Podesta added that he thinks if Clinton were to lose either or both of the first two states, there is a firewall for Clinton in March with a series of southern states where she appears to have an advantage with black voters.

“We feel good about the map,” Podesta said.

Indeed, Clinton has enjoyed a massive, roughly 40-point lead in South Carolina and in Florida in recent months – together, those states provide far more delegates than Iowa and New Hampshire. Clinton holds a strong advantage among black and Latino voters who play a bigger role in the primaries in late February and March.

However, Sanders told Fox News in an interview last week that he believes a win in Iowa, New Hampshire, or both would give him momentum and shake up the entire Democratic battle for the nomination.

At a forum aimed at young and minority voters on Monday night, the candidates found themselves defending their positions on immigration, criminal justice, gun control and abortion. Sanders needled the race front-runner.

"The inevitable candidate for the Democratic nomination may not be so inevitable today," said Sanders, when asked about his standing in Iowa.

As for his health care plan, he said during a town hall meeting that large numbers of underinsured and sky-high deductibles demand a better health care system, which he would seek through his single-payer, Medicare-for-all system.

Clinton also announced a new plan that would impose a 4 percent fee on taxpayers making more than $5 million -- an effort to match Sanders' focus on income inequality -- even as she charged him with plans to raise taxes on middle-class Americans. 

Fox News’ Ed Henry and The Associated Press contributed to this report.