Democrats and Republicans welcome Mueller as special investigator

US President Donald Trump has denounced the appointment of a special counsel to investigate his campaign's ties with Russian Federation, calling it an unprecedented "witch hunt" that "hurts our country terribly".

Reacting to the appointment, Trump earlier said a thorough investigation will prove that no such collusion took place.

It immediately escalated the legal stakes - and the potential political damage - for a president who has tried to dismiss the matter as a partisan witch hunt and a "hoax".

The revelation by Rosenstein "means that the letter that the president sent us is not accurate", Murphy said, referring to Trump's statement attributing the firing to Rosenstein's memo.

The decision to move to an independent probe came a week after Trump abruptly fired FBI Director James Comey, sparking a political firestorm as the agency was in the midst of an investigation into the Russian Federation matter. "The acting attorney general has the authority to appoint a special counsel and has done so", he added.

In naming Robert Mueller, the deputy attorney general said it was in the public interest to have an outsider.

What were the circumstances around Rosenstein's reported threat to resign if the White House continued to push the story that he initiated the move to fire Comey? Mark Warner, a Virginia Democrat running the Senate investigation said of the special counsel news: "I think it means our investigation is more important than ever". "If I remember rightly, around a month-and-a-half or two months ago, the Trump administration made a decision to ban passengers from seven Middle Eastern countries from carrying any electronic devices on board..."

"I think it divides the country", he continued.

According to sources familiar with the matter, Trump asked Comey to end the investigation into former national security adviser Michael Flynn, a request documented in a memo written by the former FBI director.

So far, no evidence of collusion has been presented, but the question has consumed the first four months of Trump's presidency and spurred multiple crises that have left the White House shell-shocked.

However, Trump later lashed out at the appointment of the special counsel in a series of tweets.

Graham said that the nature of Mueller's investigation has hurt the Senate's ability to call witnesses in their counter-intelligence probe.

President Trump released a statement saying there were no ties between his campaign and Russian Federation.

"No politician in history, and I say this with great surety, has been treated worse or more unfairly", he said on Wednesday to the graduating class of the United States Coast Guard Academy.