INDIANOLA, Iowa — As Hillary Clinton closes in on the final days of campaigning in Iowa, she launched a new broadside against her principal Democratic rival, Sen. Bernie Sanders (Vt.), over his inability to advance a single-payer health-care plan throughout his decades in Congress.

"Senator Sanders has been in Congress for 25 years," Clinton said at a rally at Simpson College on Thursday. "He never got even a single vote in the House or a single Senate co-sponsor. Not one."

"You hear a promise to build a whole new system, but that’s not what you get," she added. "You’ll get gridlock and an endless wait for advances that never come."

In recent weeks, Clinton has zeroed in on health care as one of the defining distinctions between her candidacy and that of Sanders.

Last week, her campaign initially argued that Sanders wanted to "strip" health coverage from Americans by handing control to Republicans. On Thursday, Clinton focused on the likelihood that a proposal like Sanders's would go nowhere in Congress.

"In theory, there’s a lot to like about some of his ideas," Clinton said. "But 'in theory' isn’t enough. A president has to deliver in reality."

A single-payer health-care system like the one proposed by Sanders is widely popular among Democrats. But Clinton has argued that as president, she would be best equipped to build on the existing health-care law, the Affordable Care Act, and eventually reach universal coverage by insuring more people under that law.

"I know Senator Sanders cares about covering more people, as I do," Clinton said. "But rather than build on the progress we’ve made, he wants to start over from scratch with a whole new system."

"More gridlock in Washington won’t help anyone, except maybe the Republicans," she added.

Clinton's argument against Sanders has increasingly focused on his preparedness for the job — and his ability to deal with the complex issues and political division the next president will face.

On foreign policy, Clinton said that Sanders's statements should raise concerns, particularly his recent statements on Iran.

"Senator Sanders doesn’t talk much about foreign policy, but when he does, it raises concerns because sometimes it can sound like he hasn’t really thought it through," Clinton said.

She pointed to comments Sanders made that the U.S. should normalize relations with Iran and that Iran should be invited to send more troops to Syria to join in a military coalition with Saudi Arabia.

"That is like asking the arsonist to be the firefighter," Clinton said.

Her campaign has seized on these statements to suggest that Sanders does not have a firm grip on foreign affairs.

"The challenges the president has to grapple with are beyond complicated both at home and abroad. that’s why it is the hardest job in the world," she added. "I’ve seen it up close and personal and I know what it takes."