If Elected, Trump May Not Be Able to Fulfill Big Debate Promise

— -- Federal law may prevent Donald Trump, if elected, from fulfilling a big promise he made at last night's presidential debate, according to former U.S. Justice Department chiefs from both political parties.

"If I win, I am going to instruct my attorney general to get a special prosecutor to look into [Hillary Clinton]," the Republican nominee vowed, citing her use of a private email server as secretary of state. "We're going to have a special prosecutor."

But former attorneys general under Republican and Democratic administrations said presidents don't get to decide on the appointment of a special prosecutor.

"I don't conceive of that as something that's in the authority of the president," said Michael Mukasey, who was an attorney general under President George W. Bush and has been an outspoken critic of Clinton for her use of the private server.

Mukasey and other former Justice Department heads said that the president may request a special investigator be named but that it's up to the attorney general whether to appoint one.

Federal law states, "The attorney general, or in cases in which the attorney general is recused, the acting attorney general, will appoint a special counsel when he or she determines that criminal investigation of a person or matter is warranted."

Mukasey told ABC News, "The president can say what they want to happen, but the attorney general's proper response would be, 'That's interesting. I'll take a look. But I decide that. You don't.'

"The president can [stomp] his Buster Browns on the sidewalk all he wants, but he doesn't have the authority to make a decision," he said.

Mukasey noted that it takes someone "with a spine" to make the right decision, especially considering that the president has the power to fire the attorney general.

Though the attorney general serves at the pleasure of the president, the head of the Justice Department is supposed to act with a measure of independence.

In 2004, Attorney General John Ashcroft and Deputy Attorney General James Comey — who is now the FBI director — famously refused White House efforts to get them to approve more secret surveillance by George W. Bush's administration.

Eric Holder, who served as attorney general under Barack Obama until last year, described Trump's promise as evidence that the Republican nominee is "dangerous" and "unfit" to be commander-in-chief.

"He is promising to abuse the power of the office," Holder, a vocal Clinton supporter, tweeted last night. "Be afraid of any candidate who says he will order DOJ/FBI to act on his command." When President Richard Nixon demanded that his Attorney General Elliot Richardson fire a special prosecutor appointed to investigate the Watergate break-in, Richardson "courageously resigned," Holder tweeted.

Clinton has apologized for her use of the private email server, saying it was a mistake.

Even before the FBI concluded its investigation of her use of the server and recommended no charges be filed, many prominent Republicans called for Attorney General Loretta Lynch to appoint a special counsel to take over the matter, citing potential conflicts of interest for the Justice Department.

Mukasey said a special prosecutor would likely be necessary if federal authorities in the next administration decide to re-examine Clinton's handling of classified information on her private server.

Because officials in the Justice Department and FBI participated in the initial investigation and in the decision not to bring charges, if the case is reopened, "a special counsel would be warranted," he said.

Trump said at the debate last night, "We're going to get a special prosecutor, and we're going to look into" the Clinton email matter, he said.

But if a President Trump wanted a special prosecutor, he would have to request it, and the attorney general would have to approve.