WASHINGTON, Sept 20 (Reuters) - U.S. Attorney General Merrick Garland told a House committee on Wednesday that Republican threats to defund the FBI would be "catastrophic" if carried out and that the Justice Department did not exist to do anyone's political bidding.

Garland pushed back against Republican lawmakers who have criticized the Department of Justice for its handling of the indictments of Republican former President Donald Trump and Democratic President Joe Biden's son, Hunter Biden.

"Our job is not to take orders from the president, from Congress, or from anyone else, about who or what to criminally investigate," Garland told the House of Representatives Judiciary Committee.

"I am not the president’s lawyer. I will add I am not Congress’s prosecutor. The Justice Department works for the American people."

Some of Trump's hardline Republican allies have called for a defunding of the FBI to protest its investigation into and prosecution of more than 1,140 Trump supporters who stormed the U.S. Capitol on Jan. 6, 2021, in a bid to overturn his election defeat.

Garland warned that carrying out that threat would leave the nation "naked" to everything from the "malign influence of the Chinese Communist Party" to "domestic violent extremists."

"I just cannot imagine the consequences of defunding the FBI," Garland said. "They would be catastrophic."

Wednesday marked Garland's first testimony before Congress since two historic firsts: the department's criminal charges against a former U.S. president and against a sitting president's adult child.

It also comes a week after the Republican-led House launched an impeachment inquiry into President Biden, related to Hunter Biden's foreign business dealings and as congressional inaction threatens to cause the fourth partial U.S. government shutdown in a decade beginning next month.

The White House has dismissed the impeachment probe as politically motivated and unsubstantiated. The committee's ranking Democrat, Jerrold Nadler, on Wednesday accused Republicans of wasting "countless taxpayer dollars" on investigations into Biden "to find evidence for an absurd impeachment."

Special Counsel Jack Smith, appointed by Garland last autumn, has twice secured indictments of Trump over his alleged mishandling of classified records and for his alleged efforts to overturn the results of the 2020 presidential election.

Trump, the front-runner for the 2024 Republican presidential nomination, has pleaded not guilty to those charges and to two state criminal indictments he faces in New York and Georgia.

The former president has repeatedly verbally attacked Smith, potential witnesses, and U.S. District Judge Tanya Chutkan, who is presiding over the election subversion case, saying the prosecutions he faces are politically motivated.

Republicans have also been critical of the department's handling of a five-year-long tax investigation into Hunter Biden, 53.

The younger Biden was set in July to plead guilty to two misdemeanor tax counts and to agree to enroll in a program to avert a gun charge as part of a deal with the then-U.S. Attorney for Delaware, David Weiss.

The deal collapsed after a federal judge questioned its terms.

Shortly before that, an Internal Revenue Service whistleblower who worked on the criminal tax probe also claimed that the Justice Department stymied Weiss from pursuing more serious tax charges by failing to appoint him sooner as special counsel, so that he could pursue the cases in either Washington, D.C., or Central California. Hunter Biden lives in California.

Amid mounting Republican criticism, Garland appointed Weiss as special counsel so he could continue to investigate and possibly pursue tax charges in other federal districts.

Weiss' office this month charged Hunter Biden with three counts related to purchase and possession of a firearm while he was using illegal drugs. He intends to plead not guilty.

Republicans on Wednesday grilled Garland about the Hunter Biden case.

"Has anyone from the White House provided direction at any time to you personally or to any senior officials at the DOJ regarding how the Hunter Biden investigated was to be carried out?" Republican congressman Mike Johnson asked.

"No," Garland said.

The attorney general also defended how the investigation was carried out under Weiss, saying he never "intruded" into Weiss' work and telling Congress that Weiss always had "full authority to conduct his investigation" as he saw fit and only recently sought special counsel status.