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.