By Nathan Layne

(Reuters) -Donald Trump's main rivals for the 2024 Republican presidential nomination quickly condemned the Justice Department for its move to charge him over his handling of classified documents, underscoring their fear of upsetting his core supporters.

The indictment of a former president on federal charges is unprecedented in U.S. history, a case made more extraordinary by Trump being the clear front-runner in the Republican race to challenge Democratic President Joe Biden next year.

Instead of using the indictment to undermine Trump's bid for the White House, however, Florida Governor Ron DeSantis and U.S. Senator Tim Scott were among the candidates accusing the Justice Department of political bias, highlighting a stance that has become central to many of their own campaigns.

"The weaponization of federal law enforcement represents a mortal threat to a free society," DeSantis, who is running a distant second behind Trump in the polls, wrote on Twitter. "We have for years witnessed an uneven application of the law depending upon political affiliation."

Trump was indicted by a federal grand jury for illegally retaining classified documents and for obstruction of justice.

He faces seven criminal counts, including conspiracy and obstruction of justice, and he said he was summoned to appear at the federal courthouse in Miami on Tuesday.

Former Arkansas Governor Asa Hutchinson, a long-shot Republican candidate, was the sole rival so far to outright criticize Trump. Arguing that Trump had flouted the Constitution and shown a "disrespect for the rule of law," in a statement Hutchinson called on him to end his campaign.

Former New Jersey Governor Chris Christie, who this week blasted Trump as he announced his own 2024 bid, said he wanted to see the details of the indictment before weighing in.

By and large, however, the challengers came to Trump's defense, perhaps mindful of how Trump's March indictment in New York over an alleged hush money payment to a porn star only served to boost his poll numbers.

Many Republicans viewed the prosecution as politically charged and rallied to his side.

Rivals are wary of angering Trump's base, which is thought to make up 30% of the Republican electorate and is largely unshakeable in support for Trump.

RIVALS ECHO TRUMP'S RHETORIC

They have also adopted Trump's rhetoric against the Justice Department, accusing federal prosecutors of singling out Republicans, even as they cheer on a federal investigation into Biden's son.

Scott, who is polling in the single digits, criticized what he called the "weaponization" of federal prosecutors.

"Today what we see is a justice system where the scales are weighted," he said in an interview on Fox News late on Thursday.

Vivek Ramaswamy, a venture capitalist also considered a long shot for the Republican nomination, issued a statement accusing Biden's Justice Department of unfairly targeting the former president and vowed to pardon Trump if elected.

"I never thought we’d see the day when the U.S. President deputizes the DOJ to arrest his lead rival in the middle of an election," Ramaswamy said.

Biden, who is facing his own review of his handling of classified documents, said on Thursday before the indictment news broke that the public can be confident that the Justice Department acts fairly and with independence, including in its investigation into Trump.

“I have never once, not one single time, suggested to the Justice Department what they should do or not do relative to bringing a charge or not bringing a charge,” Biden said during a White House press conference.

Federal authorities with the U.S. attorney general's office in Delaware have been probing Biden's son, Hunter, over tax-related issues since 2018. The president's son has denied wrong-doing.

On a CNN town hall earlier this week, former Vice President Mike Pence said he thought the Justice Department did not treat Trump and Biden equally in investigating the retention of classified documents and argued against indicting Trump, saying it would be too divisive for the country.

Appearing on a conservative radio show on Friday morning, Pence called for the indictment to be unsealed and for Attorney General Merrick Garland to address the nation.

While accusing the Justice Department of bias, Pence said the American people should "be able to judge for themselves whether this is just the latest incident of weaponization and politicization at the Justice Department, or if it's something different."

Chuck Coughlin, a longtime consultant for Republicans in Arizona, said he believes the cumulative effect of criminal charges will begin to take its toll on Trump's support among Republican voters.

Trump is under investigation in Georgia for allegedly trying to overturn the 2020 election in the state, and faces a separate federal probe into his alleged role in the Jan. 6, 2021 attack on the U.S. Capitol by his supporters.

If the indictments pile up, Coughlin predicts the other Republican candidates will start to argue that Trump cannot win the general election.

"There's got to be a fatigue factor there," Coughlin said. "It has to begin to create pause."

(Reporting by Nathan Layne and Dan Whitcomb; Editing by Colleen Jenkins, Kieran Murray and Daniel Wallis)