Democratic presidential candidate Bernie Sanders said Sunday that he will release his medical records, after saying that any attempt by a political rival to get them is “vile and desperate,” as the primary race tightens and becomes more hard-hitting ahead of Sunday’s debate.

Sanders has increasingly cut into the lead of front-running primary rival Hillary Clinton ahead of the debate, the final one before Iowa residents are the first to vote, in the state’s Feb. 1 Democratic and Republican caucuses.

“Of course we’re going to release our medical records, the same way that … Clinton has gotten her medical records out,” the 74-year-old Sanders said Sunday on CNN’s “State of the Union." “It is not a problem.”

The Vermont independent senator and self-proclaimed Democratic-socialist had until Sunday tried to capitalize on the reported request for his medical records by David Brock, founder of the pro-Clinton super-PAC Correct the Record.  

“They are insinuating Bernie is too old and unhealthy to be our next president,” Sanders campaign manager Jeff Weaver wrote in a fundraising email sent Saturday. 

“Bernie is in excellent health. But this personal attack is another example of a sickness in our democracy when it is so easy for millionaires and billionaires to buy up candidates and elections.”

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Even the Clinton campaign realized the political peril of questioning whether Sanders was fit enough to be president, instead of challenging him on such issues as health care, affordable college, Wall Street reform and income inequality.

“Chill out. We're fighting on who would make a better president, not on who has a better Physical Fitness Test,” Clinton campaign manager John Podesta tweeted Saturday to Brock.

The Sunday night debate will be hosted by NBC in Charleston, S.C.

Gun control has been a major issue so far in the 2016 presidential races but is expected to be an especially hot topic in this debate.

Clinton is hammering Sanders for just recently supporting a measure that would put more liability on gun manufacturers and for hesitating on whether to support efforts to close the so-called “Charleston loophole” -- which allows a gun sale to proceed if the federal background check on the buyer is not returned in three days.

Clinton and Sanders will be joined on stage by fellow Democratic candidate Martin O’Malley. The former Maryland governor is a strong proponent of tougher gun laws but is polling only at about 3 percent.

With lots of fundraising money and a last name associated with a political dynasty, Clinton, a former first lady, New York senator and secretary of state, entered the race as the clear front-runner.

However, Sanders’ progressive and inclusive message has captivated voters who appear frustrated by Wall Street and the Washington establishment controlling the direction of the country.

Sanders trailed Clinton nationally 56 percent to 31 percent in late December, but has now cut that lead to 13 percentage points, 51-to-38, according to the RealClearPolitics polls average.

He now trails Clinton by just 4 percentage points in Iowa, after being behind 56-27 in early November. And Sanders now leads Clinton by 7 percentage points in New Hampshire, which on Feb. 9 votes second in the country.

The debate will be the fourth so far for Democrats.

Sanders began the debate season by letting Clinton off easy, saying Americans were “sick and tired” of hearing about her email controversy, related to her tenure at the State Department.  

Still, the Sanders campaign doesn’t appear ready to launch a major attack this time.

 “We don’t feel like we have to go in there and heat it up,” Sanders adviser Tad Devine told The Hill newspaper.