WASHINGTON (Reuters) - A key Republican senator said on Thursday that acting Attorney General Matthew Whitaker, who now oversees a probe of whether President Donald Trump’s campaign colluded with Russia during the 2016 election, told him he had no concerns about the special counsel leading the investigation.

Acting Attorney General Matthew Whitaker stands during a presentation of the colors at the Annual Veterans Appreciation Day Ceremony at the Justice Department in Washington, U.S., November 15, 2018. REUTERS/Yuri Gripas

Senator Lindsey Graham, after a meeting with Whitaker, said he was confident there would be no interference in the probe led by Special Counsel Robert Mueller.

“He’s seen nothing out of bounds or no concerns at all about Mr. Mueller,” Graham, a senior member of the Senate Judiciary Committee and a close ally of Trump, said in comments confirmed by his spokesman.

He said he saw no need for Whitaker to recuse himself as critics have demanded, the spokesman confirmed.

Democrats and others fear Whitaker’s appointment could jeopardise Mueller’s probe of Russia’s role in the 2016 U.S. election.

Whitaker, a Trump loyalist and a former U.S. attorney in Iowa, had criticized the Mueller probe as too far-reaching before he was appointed by Trump last week to run the Justice Department.

Trump has denied that his 2016 presidential campaign colluded with Russia, and calls the Mueller probe a witch hunt.

U.S. intelligence agencies have concluded that Russia interfered in the 2016 election to help Trump by undermining Democratic candidate Hillary Clinton. Russia has denied any meddling in the election.

Congressional Democrats on Thursday dismissed as “fatally flawed” a Justice Department legal opinion defending Trump’s decision to appoint Whitaker as acting attorney general to replace ousted Jeff Sessions.

The top Democrats on the House and Senate Judiciary Committees and the House Intelligence Committee said in a joint statement that the opinion released on Wednesday twists the language of the U.S. Constitution and ignores an existing law governing succession at the Justice Department.

In a preview of the opposition Trump can expect in January, when a new Democratic majority takes control of the House of Representatives, the Democrats said Whitaker should not have been appointed because he has not been confirmed by the Senate.

“This will embolden the future use of temporary appointments for illegitimate purposes... This can’t be allowed to stand,” said the statement from House Judiciary Committee Democrat Jerrold Nadler, House Intelligence Committee Democrat Adam Schiff and Senate Judiciary Committee Democrat Dianne Feinstein.

Nadler and Schiff are expected to become chairmen of their committees in the next Congress.

Trump has repeatedly attacked the investigation.

“The inner workings of the Mueller investigation are a total mess. They have found no collusion and have gone absolutely nuts,” the president said in a tweet on Thursday. “They are a disgrace to our Nation and don’t ... care how many lives the ruin.”

The Mueller investigation has led to criminal charges against several former Trump aides.

The Justice Department’s opinion concluded that Whitaker’s appointment was allowed under a 1998 law called the Federal Vacancies Reform Act. It rejected views that the move ran afoul of the U.S. Constitution’s requirement for “principal officers” to be confirmed by the Senate.

The Justice Department also cited an 1866 example of an acting attorney general being appointed without Senate confirmation, which Democrats pounced on.

“That was for six days in 1866 - the year after the Civil War ended, four years before the Justice Department’s founding and a century before the (Justice Department) succession law was enacted,” the Democrats said.