Hillary Clinton. REUTERS/Aaron P. Bernstein Hillary Clinton is beginning to dedicate more and more time to explaining why she believes that presidential rival Sen. Bernie Sanders (I-Vermont) isn't qualified for the presidency.

Clinton, the Democratic front-runner, sharpened her attacks on her chief primary rival during a speech in Iowa Thursday, taking aim at him on healthcare and foreign policy and questioning whether his plans were even practical.

Of Sanders' universal healthcare proposal, she said that while it was great "in theory," it wouldn't be able to pass through Congress and would create a "divisive debate" that would end in gridlock.

"I know Sen. Sanders cares about covering more people, as I do. Rather than build on the progress we've made, he wants to start over from scratch with a whole new system. In theory, there's a lot to like about some of his ideas. But in theory isn't enough. A president has to deliver in reality," Clinton said.

She then took a shot at what she suggested was a lack of legislative prowess.

"Sen. Sanders has been in Congress for 25 years," she said. "He introduced his healthcare plan nine times, but he never got a single vote in the House or a single Senate cosponsor. You hear a promise for a whole new system. But that's not what you'll get. You'll get gridlock and an endless wait for advances that will never come. The people who I've met can't wait."

As Sanders has expanded his lead in New Hampshire and caught up with Clinton in Iowa, Clinton and her campaign have been more forceful in their criticism.

Clinton's campaign held a conference call with reporters Thursday to "question" Sanders plans for taking on the terrorist group ISIS and dealing with Iran, with whom the US and other world powers brokered a landmark nuclear deal last year.

Jake Sullivan, a senior Clinton campaign policy adviser, took aim at Sanders' comments during Sunday's Democratic debate, during which he suggested he would favor normalizing relations with Iran. He has also suggested that Iran should deploy more troops to Iraq.

"That would be like inviting one of the arsonists to join the firefighters," Sullivan said.

Sullivan's comments echoed a common theme of recent days: casting Sanders as a foreign-policy lightweight.

In a letter released by the Clinton campaign on Tuesday, 10 top diplomats questioned Sanders on the issues of ISIS and Iran.

"We are concerned that Sen. Sanders has not thought through these crucial national security issues that can have profound consequences for our security," the letter said.

“His lack of a strategy for defeating ISIS - one of the greatest challenges we face today — is troubling. And the limited things he has said on ISIS are also troubling."

For his part, Sanders has fought back, citing Clinton's vote in favor of the Iraq War as an example of why her foreign-policy experience is undermined by her judgment in some instances.

After an Iowa town-hall event Tuesday, Sanders made the point to reporters that former Vice President Dick Cheney, who served under President George W. Bush, also had foreign-policy experience.

“Experience is important. Dick Cheney had a lot of experience," Sanders said, according to The Washington Post. "A whole lot of people have experience but do not necessarily have the right judgment. I think I have the right judgment to conduct sensible foreign policy.”

Pamela Engel contributed reporting.