"That means the House should initiate impeachment proceedings against the President of the United States," Warren, a senator from Massachusetts, tweeted.

Mueller, who investigated whether Trump's campaign coordinated with Russia during the 2016 election, found that Trump made numerous attempts to interfere with the investigation but was largely foiled by those around him.

He found no evidence of a direct conspiracy between Russia and the Trump campaign and made no verdict on obstruction of justice.

Republican Senator Romney said the conduct by Trump and his campaign outlined in the report left him "sickened" and "appalled" -- marking one of the first prominent Republicans to make such critical comments of the president in the aftermath of the blockbuster investigation.

Romney offered his reaction on Twitter after reading the full 448-page report, writing that the level of lies and dishonesty outlined in the report left him with the realisation of "how far we have strayed from the aspirations and principles out the founders."

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"I am sickened and the extent and pervasiveness of dishonesty and misdirection by individuals in the highest office of the land, including the President," wrote Romney, a one-time presidential nominee who has repeatedly clashed with Trump over the years.

"I am also appalled that, among other things," fellow citizens working in a campaign for the president welcomed help from Russia -- including information that had been illegally obtained."

Congressional Democrats took legal action on Friday to gain access to all of Mueller's evidence from his inquiry into Russian meddling in the 2016 election, as the probe's findings dented President Donald Trump's poll ratings.

The number of Americans who approve of Trump dropped by 3 percentage points to the lowest level of the year following the release of a redacted version of Mueller's report on Thursday, according to a Reuters/Ipsos online opinion poll.

Mueller did not establish the Trump campaign coordinated with Russians but did find "multiple acts by the President that were capable of exerting undue influence over law enforcement investigations."

While Mueller ultimately decided not to charge Trump with a crime such as obstruction of justice, he also said the investigation did not exonerate the president, either.

U.S. House of Representatives Judiciary Committee Chairman Jerrold Nadler, a Democrat, issued a subpoena to the Justice Department to hand over the full Mueller report and other relevant evidence by May 1.

"My committee needs and is entitled to the full version of the report and the underlying evidence consistent with past practice. The redactions appear to be significant," Nadler said in a statement.

Other 2020 Democratic presidential candidates, while supportive of the idea of impeachment, were more circumspect in their responses.

Former Housing and Urban Development Secretary Julian Castro said on CNN that it would be "perfectly reasonable for Congress to open up those proceedings".

Both Washington Governor Jay Inslee and congressman Eric Swalwell said the question of impeachment should not be taken off the table.

AP, Reuters, TNS