WASHINGTON, May 27 (Reuters) - A moderate Democrat made an eleventh-hour appeal on Thursday to Republicans in the U.S. Senate to support a commission to investigate the deadly Jan. 6 attack on the Capitol, but the idea seemed to lack critical support ahead of a vote.

"There is no excuse for any Republican to vote against this commission since Democrats have agreed to everything they asked for," Senator Joe Manchin wrote on Twitter.

Supporters of then-President Donald Trump stormed the Capitol building on Jan. 6 while Congress was certifying the election victory of Democratic President Joe Biden. Five people died in the violence.

With a procedural Senate vote expected as early as Thursday, just three Republicans have explicitly said they will vote to allow floor consideration of a measure - negotiated by members of both parties in the House of Representatives - creating an independent commission to investigate the attack.

That is well short of the 10 Republicans needed under the Senate's rules, which require 60 votes for a bill to advance. Democrats back establishment of an investigative panel, and the Senate is divided 50-50 along partisan lines.

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Minority Leader Mitch McConnell and other senior Republicans have argued that ongoing probes by two Senate committees, as well as the investigations of numerous prosecutors, are sufficient to examine those events.

Manchin poured scorn on those arguments, saying McConnell's opposition was aimed at protecting Republicans from potential damage in congressional elections next year.

"McConnell has made this his political position, thinking it will help his 2022 elections. They do not believe the truth will set you free, so they continue to live in fear," he wrote.

Manchin, of West Virginia, has made it one of his missions in Congress to seek bipartisanship on various issues. He has split with his own party numerous times.

The bill establishing the commission passed the Democratic-controlled House last week with support from 35 Republicans who defied their party's leadership in voting for the measure. The legislation would require that the commission issue a final report by Dec. 31.

The mother of Brian Sicknick, a Capitol Police officer who died a day after battling the Jan. 6 mob, appealed for support for a commission in a meeting on Thursday with senators on both sides of the issue.

Emerging from a session with Senator Mitt Romney, one of the handful of Republicans who support the panel, Gladys Sicknick said she was speaking out because "I just couldn't stay quiet any more." Her son Brian, 42, died of a stroke a day after fighting with the mob that smashed windows, attacked officers with poles and bear spray and sent lawmakers running for cover.

After meeting with Sicknick, Republican Senator Ron Johnson said he still did not see the value of a commission although he condemned the violence on Jan. 6.

“All those who engaged in those repulsive acts should be prosecuted to the full extent of the law," Johnson said in a statement.

A U.S. judge said there is a risk that Trump's supporters could carry out attacks similar to the assault on the Capitol, noting the former president's "near-daily fulminations" about his election loss.

(Reporting by Susan Cornwell and Susan Heavey; Editing by Scott Malone and Sonya Hepinstall)