The Republican presidential candidates are seizing on the “top secret” Hillary Clinton emails in the final weekend before the Iowa Caucus, trying to slow their top Democratic rival by arguing her mishandling of the messages makes her unfit to be president.

“Hillary Clinton is a major national security risk. Not presidential material!” GOP frontrunner Donald Trump tweeted after the State Department said Friday that it will withhold 22 emails from Clinton's correspondence as secretary of state because they are classified “top secret.”

Trump and the 10 other Republican candidates are barnstorming across Iowa this weekend ahead of the Iowa Caucus on Monday, the first-in-the-nation balloting in the 2016 presidential race.

Clinton, the Democratic frontrunner, revealed last spring that she used a private server and email accounts for official business when she was the country’s top diplomat.

The State Department has in compliance with a federal court order released thousands of page of Clinton emails, with classified information redacted. However the agency said Friday that the 22 top-secret messages, in seven email chains, will not be released.

The intelligence community has deemed some of Clinton’s emails “too damaging" to national security to release under any circumstances, a U.S. government official close to the ongoing review told Fox News. 

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“The new e-mail release is a disaster for Hillary Clinton,” Trump also tweeted. “At a minimum, how can someone with such bad judgement be our next president?”

Clinton has repeatedly said she never sent classified information through her private accounts. Her campaign on Friday questioned the secrecy of the messages and called for the State Department to release them.

The FBI is investigating the matter, which has raised questions about how federal agencies have different rules for classifying information and whether some of the emails were marked classified after the fact.

Vermont Sen. Bernie Sanders, Clinton’s top primary challenger, has declined to talk about the emails.

There is a “legal process in place, which should proceed and not be politicized," he said Friday.

During the first Democratic debate last year, Sanders famously dismissed the issue by saying, "the American people are sick and tired of hearing about your damn emails!"

He trails Clinton by rough 5 percentage points in Iowa but leads her by roughly 15 points in New Hampshire, which votes Feb. 9, according to the RealClearPolitics poll averaging.  

Florida Sen. Marco Rubio, running third in most GOP polls behind Texas Sen. Ted Cruz, told an audience in Clinton, Iowa, that such sensitive emails being on Clinton’s server is “not acceptable.”

Cruz suggests that Clinton’s use of the private server and emails now appears “far more serious” than previously thought and that the most recent revelations put her candidacy into more peril.

Cruz, who has 15 events in Iowa before the caucus, is also raising questions about whether the Justice Department, run by the Obama administration, would indeed indict Clinton, if the investigation leads to that decision.

“There is an acceptance that the enforcement of criminal justice is decided not by the laws of this country, but by some political hack in the West Wing of the White House.That is not how our Constitution is meant to operate,” Cruz said on the Hugh Hewitt radio show.

“If she is indicted, it is difficult to see how she could successfully run for president. I would put nothing past the gall and audacity of the Clintons to try. But even the Democratic Party, I would find it hard to believe that they would be eager to nominate someone who is under indictment and could well face felony incarceration.”

Fox News' Catherine Herridge and Pamela K. Browne and The Associated Press contributed to this report.