(CNN) Bernie Sanders' elevated status in the Democratic presidential race -- and the increasing pressure he faces as a result -- is on full display as Iowa voters prepare to cast the first decisions of 2016.

Polls show him running neck-and-neck with Hillary Clinton there, upending convention wisdom and the expectations of the Clinton camp.

One of his rivals on the Republican side paid him a compliment of sorts by bashing him during the debate on Fox News.

Marco Rubio laid into Sanders in response to a moderator's question, knocking his Senate colleague over his democratic socialist outlook. "We don't want to be Sweden. We want to be the United States of America."

Perhaps sensing the previously unthinkable -- a victory in Iowa -- the Democratic candidate himself spent the day cleaning up some loose ends before the caucuses. He decided to co-sponsor a measure placing gun manufacturers on the hook in civil lawsuits, completing his reversal on the issue. He released a doctor's note stating he is in good health. And his campaign touted a backup system in place to tally caucus results, amid some stated worries that Microsoft may tinker with results.

Sanders appeared unusually tense Thursday, when he blew up at a reporter who asked if he would be busing in students to the Iowa caucuses. Sanders first muttered "Jesus Christ," then attacked David Brock, the Democratic operative and close Clinton ally who has been hammering away at him.

"Based on what do they say that? Based on David Brock's long history of honesty and integrity?" Sanders said. "The man who tried to destroy Anita Hill. Is this where this is coming from?" Sanders said. "Every one of you knows -- and you know it -- that every day you're being flooded by all this negative stuff from the Secretary Clinton Super PAC. That's the first I ever head of that. I don't want my integrity and honesty to be impugned. I have no idea who said this. This is a lie, an absolute lie."

Mo Elleithee, former communications director for the Democratic National Committee and a Clinton staffer in her 2008 campaign, said the last week before Iowa is a key test for every candidate.

"There is not a more tense time in a campaign than the final days, and I actually think it's particularly so in Iowa because the stakes are so high," said Elleithee, who now runs the Georgetown Institute of Politics and Public Service. "Having been there, it is gut-wrenching, stressful, straining. Half of my gray hairs I think I got in the week before Iowa."

A few hours after Sanders' outburst, his campaign released a sharp ad, attacking Clinton for receiving speaking fees from Goldman Sachs, although it never mentioned her by name.

It was a burst of political housecleaning, trying to take care of a series of questions that have been hovering over his head for months. He only recently said he would release his health records, after months of pestering (and elective hernia surgery.) He also signed on as a co-sponsor for legislation that would repeal immunity for gun manufacturers.

One question still looming is whether the two will have additional debates. Sanders said Thursday he would do a debate next week only if it was first sanctioned by the Democratic Party.

"She can't just dictate the rules of the game. These were rules, I had nothing to do with, established by the DNC, which most people perceived to be sympathetic to her. Now she's falling apparently behind in New Hampshire and wants to change the rules," Sanders told CNN Thursday.

Clinton countered by telling CNN's Wolf Blitzer on "The Situation Room" that she would indeed agree to more debates -- but declined to get into specifics.

"We have another one shortly after New Hampshire on the calendar, and I've said we should start looking for dates to get one scheduled. I'm perfectly fine with that," she said. "But first things first: We have to agree that we're going to debate in New Hampshire. Both (former Maryland Gov. Martin O'Malley) and I have agreed and we're waiting for the senator to decide to join us."

Elleithee said he sees Sanders in a similar position to where Clinton was in 2008, tensing up. He sees Clinton easing up now, in this final week, but noted she will still have to overcome the energy behind Sanders.

One of the final pieces Sanders let out Thursday was a clean bill of health from the Capitol Hill attending physician Brian P. Monahan who has treated Sanders for the last 25 years, since he began serving in the House.

It didn't carry quite the same level of praise as Donald Trump's health assessment, but it was a key step for the man who could become the oldest person to assume the presidency -- he would turn 75 on Inauguration Day.

The one thing he doesn't have to worry about from Iowa is getting any gray hairs.