Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE is attacking Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE from the right on healthcare ahead of next month’s Democratic caucuses in Iowa.

The Democratic presidential front-runner’s campaign is blasting her chief rival’s failure to address how he’d pay for a single-payer healthcare system. She argued he would have to impose steep tax increases to come up with the money.

“We think the voters in Iowa should be able to hear how Sen. Sanders would pay for that proposal,” Brian Fallon, the campaign’s national spokesman, said Wednesday in a phone briefing on the issue.

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Clinton’s senior policy adviser, Jake Sullivan, said on the call that Sanders is refusing to disclose his plan’s price tag because the details are “politically difficult,” with potentially trillions of dollars of new spending and hefty new tax hikes.

Sanders did not mention healthcare in a trillion-dollar tax outline released Wednesday, and his campaign has not said when those details would be revealed.

Clinton has sharpened her criticism of Sanders, overall, as polls have shown him taking a slim lead this week. A poll released Wednesday by Quinnipiac University showed that Sanders would earn support from 49 percent of Democratic voters in Iowa compared to Clinton’s 44 percent.

The Sanders campaign — as well as some progressive allies — has fought back with full force, accusing Clinton of using “Karl Rove tactics” that she once condemned as a presidential candidate in 2008. In that race, she said Democratic attacks on universal healthcare help Republicans while undermining “core Democratic values.”

The candidates’ tense back-and-forth on healthcare took off on Tuesday after Hillary Clinton’s daughter, Chelsea Clinton, said Sanders was trying to “dismantle” ObamaCare and its programs. She didn’t mention the single-payer system is intended to replace, not simply eliminate, ObamaCare and that it would also replace premiums.

In a response to those comments Tuesday night, Sanders said Chelsea Clinton was “absolutely wrong” and had not read his healthcare plan.

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Hillary Clinton has launched a full-court press in Iowa, with Chelsea and former President Bill Clinton both campaigning there this week.

Since the start of their campaigns, Clinton and Sanders both have outlined similar proposals for healthcare problems such as rising prescription drug costs, though Sanders has continued to advocate for his single-payer plan.

Clinton’s new attacks on Sanders’s “Medicare for all” system is perplexing some progressives.

Charles Chamberlain, president of the grassroots group Democracy for America, called out Clinton for her “right-wing attacks” that he said “have no place in a Democratic primary.”

Sullivan, the policy adviser, dismissed claims that Clinton was using a Republican-style argument against single-payer. He argued that Clinton’s calls to build on the Affordable Care Act — “and not to be starting this whole thing over” — were consistent with the views of President Obama and national Democrats.

“I don’t believe we’re hitting anybody from the right,” Sullivan said.

Still, Clinton’s campaign will have to tread carefully with its new line of attack: A single-payer system, like those of Sweden or Canada, is supported by many Democrats.

Aides acknowledged the popularity of the plan in their call on Wednesday, arguing instead that Sanders is letting voters down by refusing to release details.

“Clinton absolutely respects Democrats who support the principle of single-payer,” Sullivan said, adding that “now is not the moment to plunge this country into a debate on healthcare.”

“Our task now is to defend the Affordable Care Act against Republicans who are persistently voting to repeal it, and then to build on it,” he said.

This story was updated at 7:54 p.m.