WASHINGTON, April 18 (Reuters) - Special Counsel Robert Mueller's long-awaited report on Russia's role in the 2016 U.S. election will be released on Thursday, providing the first public look at the findings of an inquiry that has cast a shadow over Donald Trump's presidency.

Attorney General William Barr's planned release of the nearly 400-page report comes after Mueller wrapped up his 22-month investigation last month into the Trump campaign's contacts with Russia and questions about obstruction of justice by the president.

Its disclosure, with portions expected to be blacked out by Barr to protect some sensitive information, is certain to launch a new political fight spilling into the halls of Congress and the 2020 presidential campaign trail, as Trump seeks re-election in a deeply divided country.

The release marks a watershed moment in Trump's presidency, promising new details about some of the biggest questions in the probe, including the extent and nature of his campaign's contacts with Russia and actions Trump may have taken to hinder the inquiry including his 2017 firing of FBI Director James Comey.

It also may deepen an already bitter partisan rift between Trump's fellow Republicans, most of whom have rallied around the president, and his Democratic critics, who will have to decide how hard to go after Trump as they prepare congressional investigations of his administration.

Barr said he would hold a news conference at 9:30 a.m. (1330 GMT) on Thursday to discuss the report, along with Deputy Attorney General Rod Rosenstein, who appointed Mueller as special counsel in May 2017.

Copies of the report will be delivered to Capitol Hill more than an hour later, between 11 a.m. and noon (1500-1600 GMT), a senior Justice Department official said. The delay in seeing the report sparked Democratic complaints that Barr, a Trump appointee, wanted to shape the public's views during his news conference before others had a chance to draw their own conclusions.

Early on Thursday, top congressional Democrats called on Mueller to testify publicly in Congress about his investigation, criticizing Barr's rollout of the report.

"We believe the only way to begin restoring public trust in the handling of the Special Counsel's investigation is for Special Counsel Mueller himself to provide public testimony in the House and Senate as soon as possible," House Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer said in a statement.

The spokesman for the special counsel's office declined to comment.

Mueller's investigation, which Trump has called a "witch hunt," raised questions about the legitimacy of Trump's presidency and laid bare what the special counsel and U.S. intelligence agencies have described as a Russian operation to derail Democrat Hillary Clinton's candidacy and elevate Trump, the Kremlin's preferred candidate.

Some Democrats have spoken of launching impeachment proceedings against Trump in Congress, allowed under the U.S. Constitution to remove a president from office for "treason, bribery, or other high crimes and misdemeanors," but top Democrats have been notably cautious.

Mueller charged 34 people and three Russian companies. Those who were convicted or pleaded guilty included figures close to Trump such as his former campaign chairman Paul Manafort, personal lawyer Michael Cohen and national security adviser Michael Flynn.