Senate Democrats plan to introduce legislation “to ensure that a truly independent prosecutor can be appointed” to investigate alleged ties between Moscow and associates of President Donald Trump, Senator Dianne Feinstein has said, in the wake of Mr Trump’s firing of FBI Director James Comey.

The bill, which Ms Feinstein is working on with Senator Richard Blumenthal, could resemble an “independent counsel” statute that Congress passed in response to the Watergate scandal. That statute required a panel of three federal judges to appoint an independent counsel when there was credible evidence that a president or high-level executive official may have violated the law. Congress let the independent counsel law expire in 1999.

“It was clear the FBI was taking its job seriously, and a substantial investigation was underway,” Ms Feinstein said, and the timing of Mr Comey's dismissal “could implicate” the Trump administration.

Mr Blumenthal told the Atlantic that the legislation calls for special counsel appointed by a three-judge panel.

“However, while we work on that legislation, I want to renew my call to have a special prosecutor appointed to oversee the Russia investigation,” Ms Feinstein said.

Other top Democrats are also asking Deputy Attorney General Rod Rosenstein to appoint a special prosecutor, despite White House spokesman Sean Spicer telling reporters “there is clearly at this point no evidence of a reason” for one, as “you have a system that's working”. Senate Majority Leader Mitch McConnell has also opposed the idea.

Attorney General Jeff Sessions cannot appoint a special prosecutor because he recused himself from the Russia investigation after he failed to disclose meetings that he had with the Russian ambassador to the US, Sergei Kislyak.

In announcing the president’s decision to fire Mr Comey, the White House said the president acted based on the recommendations of both Mr Rosenstein and Mr Sessions.

Mr Rosenstein had written in a memo to Mr Sessions that he could not defend the FBI Director’s “refusal to accept the nearly universal judgment that he was mistaken” in his handling of the Hillary Clinton email investigation.

On the Senate floor, Democratic leader Charles Schumer said “there is little reason to think that Mr Rosenstein’s letter is the true reason that President Trump fired director Comey.”

“Why?” Mr Schumer added. “Because if the administration truly had objections to the way Director Comey handled the Clinton investigation, they would have had them the minute the president got into office. But he didn’t fire Director Comey then. The question is: Why did it happen last night?”

Mr Schumer pointed out that Mr Rosenstein said during his Senate confirmation hearing in March that he would be willing to appoint a special prosecutor at the appropriate time.

“If there was ever a time when circumstances warranted a special prosecutor, it is right now,” Mr Schumer said.