Illinois Sen. Mark Kirk is the latest Republican to back a revised criminal justice reform package that has bipartisan support but might fail to receive a vote due to the concerns of law-and-order conservatives.

"The Left and Right have come together here, and as a centrist, I will create more momentum for this bill," Kirk said in a Tuesday statement after announcing his support in a last-minute press conference at the Capitol.

Momentum is exactly what Republican proponents of the bill want from Kirk. Senate Majority Whip John Cornyn, Senate Judiciary Committee Chairman Chuck Grassley and Tea Party Sen. Mike Lee of Utah have been negotiating with Democrats to revise the bill in a way that would assure Republicans that the sentencing reforms wouldn't allow violent criminals out of prison.

A majority of senators supported the original bill, but less than one-third of the Republican majority agreed to co-sponsor the bill. Senate Majority Leader Mitch McConnell dislikes holding votes on hot-button issues that divide Republicans, so the negotiators have been working on changes that might coax some of the critical Republicans on board without losing the support of the Democratic minority.

"It's a good middle ground that people should be more comfortable with," a GOP Senate aide said Monday while discussing the revisions, which were first reported by the Washington Examiner. "There is no retroactive relief for any offender who is convicted of a serious violent felony."

Opponents of the bill are skeptical, but most are keeping their powder dry until after the new version is officially announced. Kirk's support could prove particularly helpful to the would-be reformers if he can convince Republican allies that the legislation would boost his chances of winning a difficult re-election campaign.

Kirk, the most vulnerable Republican in the Senate, required changes to the legislation that could raise his stock in his home state.

"We are spending about $60,000 per prisoner every year in Illinois to incarcerate individuals who leave prison more dangerous than when they arrived — everyone knows this system is broken," he said in Tuesday's statement. "My changes to this bill direct savings to fight gangs, which are too often taking children away from their families in Chicagoland."