Editor’s note: This story, which was published on the morning after Donald Trump’s victory in the South Carolina primary, incorrectly reported that former Massachusetts Gov. Mitt Romney had decided to endorse Sen. Marco Rubio (R-Fla.) for president.

The story came out of conversations with two high-level Republican campaign sources, both of whom told HuffPost that Romney’s endorsement of Rubio was a done deal. Several HuffPost editors talked through the reporting and sourcing and felt confident enough to run the story.

Romney had let it be known privately that he favored Rubio -- a position communicated to multiple senior aides for Republican presidential candidates, who anticipated that Romney would endorse Rubio.

But Romney ultimately decided that he could be more effective by becoming an “independent voice” advocating against Donald Trump, according to a high-level Republican familiar with his thinking.

COLUMBIA, S.C. -- In a major development in the Republican presidential race, 2012 GOP nominee Mitt Romney will endorse Sen. Marco Rubio (R-Fla.) for president, The Huffington Post has learned from two Republican sources.

Details of Romney's endorsement were still being worked out as of Sunday morning. The sources said that the former Massachusetts governor had been eager to provide his backing to Rubio for days but had hesitated, due to his respect for former Florida Gov. Jeb Bush (R), who suspended his campaign on Saturday night following his dismal fourth-place showing in the South Carolina primary.

Romney spent the day in Utah on Saturday and could potentially be in position to bestow his blessing upon Rubio before Tuesday's Nevada Republican caucuses--a contest that Romney won easily in both 2008 and 2012.

Romney’s backing will provide Rubio with the highest-profile endorsement of the 2016 race and is the clearest signal yet that the party’s establishment is ready to coalesce around the Florida senator as its last best chance to defeat GOP front-runner Donald Trump.

Romney has long held Rubio in high regard. The first-term senator made the vice-presidential short list for Romney's 2012 run and -- along with New Jersey Gov. Chris Christie -- spent the night at Romney’s New Hampshire vacation home over Fourth of July weekend last year.

On the heels of Trump’s resounding victory in Saturday’s South Carolina primary, and with the pivotal March 1 Super Tuesday contests looming, the clock is ticking ever faster for anyone to stop the front-runner on what now looks to be a clear path to the Republican nomination.

With Bush’s candidacy over and Ohio Gov. John Kasich having uncertain longterm prospects, Romney’s endorsement of Rubio could scarcely come at a more opportune time for the Florida senator.