(Adds Barr and possible Trump press conferences in paragraphs 2-3 and upcoming Barr congressional testimony in paragraph 10)
By David Morgan and Sarah N. Lynch
WASHINGTON, April 17 (Reuters) - Congressional Democrats could move forward quickly - as early as Monday - with subpoenas to obtain Special Counsel Robert Mueller's full report on Russian interference in the U.S. 2016 election, after Attorney General William Barr releases a version on Thursday that may have significant portions blacked out.
The release of the report will represent a milestone in Donald Trump's tumultuous presidency. Barr said he would hold a news conference at 9:30 (1330 GMT) on Thursday to discuss the report, along with Deputy Attorney General Rod Rosenstein, who appointed Mueller as special counsel in May 2017.
Trump, who went to Twitter to renew his attacks on the special counsel's investigation and the FBI, told a radio interviewer that he may hold a news conference following Barr's.
When the report is released, close attention will be given not only to potential new details on the Trump campaign's contacts with Russia and the question of whether the Republican president acted to impede the inquiry, but also on how much Barr elects to withhold.
The Democratic-led House of Representatives Judiciary Committee voted on April 3 to authorize its chairman, Jerrold Nadler, to issue subpoenas to the Justice Department to obtain Mueller's unredacted report and all underlying evidence, as well as documents and testimony from five former Trump aides.
A source familiar with the matter said Nadler could issue subpoenas as early as Monday. The source, speaking on condition of anonymity, said Friday would be too early for subpoenas unless the entire report were to be blacked out.
"Chairman Nadler has said that subpoenas could come very quickly if we do not receive the full, unredacted report with the underlying evidence from DOJ. We will have to see what comes out on Thursday," committee spokesman Daniel Schwarz said in a statement, using an acronym for the Justice Department.
The department this week said the report would be released on Thursday to both Congress and the public, a day before the major religious holidays of Good Friday and Passover.
Barr, who has broad authority to decide how much of the report to release, has promised to be as transparent as possible, but told lawmakers he would redact four categories of content: secret grand jury information, intelligence-gathering sources and methods, information relating to active cases and information could affect the privacy of "peripheral third parties" who were not charged.
Barr is expected to testify on the Mueller report before the Senate Judiciary Committee on May 1 and the House Judiciary Committee on May 2.
The redactions, to be color coded to reflect the reason they were omitted from the final report, have Democrats seeing red. They have expressed concern that Barr, a Trump appointee named after the president fired former Attorney General Jeff Sessions, could black out material to protect the president.
Mueller on March 22 submitted to Barr a nearly 400-page report on his 22-month investigation into whether the Trump campaign worked with Moscow to sway the election in his favor, and whether Trump committed obstruction of justice with actions to impede the inquiry.
In a letter to lawmakers two days later, Barr said Mueller did not find that members of Trump's campaign engaged in a criminal conspiracy with Russia. Barr said he determined there was not enough evidence to establish that Trump committed the crime of obstruction of justice, though Mueller did not exonerate Trump on obstruction.
Since then, Trump has set his sights on the FBI, and accused the Justice Department of improperly targeting his campaign. Last week, Barr told a U.S. Senate panel he believed "spying" did occur on Trump's campaign, and he plans to investigate whether it was properly authorized.
A federal judge criticized Barr during a Tuesday hearing on a Freedom of Information Act lawsuit demanding access to the Mueller report, according to media reports.
"The attorney general has created an environment that has caused a significant part of the public to be concerned about whether or not there is full transparency," U.S. District Court Judge Reggie Walton was quoted as saying.
Walton said he could ask to review the full document after a redacted version is released, but denied a request by a media outlet, Buzzfeed News, to speed up the process.
(Reporting by David Morgan and Sarah N. Lynch; Writing by Doina Chiacu; Editing by Will Dunham)
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