The chairman of the US House of Representatives Judiciary Committee has issued a subpoena for special counsel Robert Mueller’s report into Russian meddling in the 2016 election and the underlying materials.

Democratic Representative Jerry Nadler of New York is asking for the information by May 1. That is also the day when Attorney General William Barr is scheduled to give evidence before a Senate committee and one day before Mr Barr is set to appear before Mr Nadler’s committee. Mr Nadler has also summoned Mr Mueller to give evidence.

The subpoena is the opening shot in what could be a lengthy legal battle as Congress escalates its investigation of US President Donald Trump.

If the Justice Department does not respond, Democrats could hold officials in contempt of Congress or eventually fight the battle in court.

Four pages of the Mueller Report
Four pages of the Mueller Report (Cliff Owen/AP)

The Judiciary Committee voted 24-17 earlier this month to give Mr Nadler permission to issue subpoenas for the final report, its exhibits and any underlying evidence or materials prepared for Mr Mueller’s investigation.

Mr Nadler said: “It now falls to Congress to determine the full scope of that alleged misconduct and to decide what steps we must take going forward.”

While Mr Mueller declined to prosecute Mr Trump on obstruction of justice, he did not exonerate the president, all but leaving the question to Congress.

Mr Mueller’s report provides fresh evidence of Mr Trump’s interference in the Russia investigation and challenges Congress to respond. The risks for both parties are clear if they duck the responsibility or prolong an inquiry that, rather than coming to a close, may be just beginning.

“My committee needs and is entitled to the full version of the report and the underlying evidence consistent with past practice,” Mr Nadler said.

Mr Barr sent Congress a redacted version of the report, blacking out several types of material, including classified information, material pertaining to ongoing investigations and grand jury evidence.

Mr Nadler said he is open to working with the department “to reach a reasonable accommodation for access to these materials, however I cannot accept any proposal which leaves most of Congress in the dark, as they grapple with their duties of legislation, oversight and constitutional accountability”.

Republicans are eager to move beyond what Mr Trump calls the “witch hunt” that has overshadowed the party and the presidency. While Democrats say Mr Mueller’s findings are far more serious than initially indicated in Mr Barr’s four-page summary last month, they have been hesitant to pursue the ultimate step, impeachment proceedings, despite pressure from the left flank of the party to begin efforts to try to remove the president from office.

House Speaker Nancy Pelosi, travelling on a congressional trip to Ireland, said in a joint statement with Senate Democratic leader Chuck Schumer that Mr Mueller’s report revealed more than was known about the obstruction question.

“As we continue to review the report, one thing is clear: Attorney General Barr presented a conclusion that the president did not obstruct justice while Mueller’s report appears to undercut that finding,” they said.

Later, in a letter to House Democrats, Ms Pelosi vowed: “Congress will not be silent.”