Report: Romney to endorse Rubio

Former GOP presidential nominee Mitt RomneyWillard (Mitt) Mitt RomneyPoll: Support growing for Utah's Medicaid expansion ballot measure Arizona Dems hope higher Latino turnout will help turn the state blue Trump changes tone on Saudi Arabia amid mounting pressure MORE will endorse Marco RubioMarco Antonio RubioO'Rourke's rise raises hopes for Texas Dems down ballot Meghan McCain calls Russian attacks against her father the 'highest compliment' to her family The Memo: Saudi storm darkens for Trump MORE’s White House bid, The Huffington Post reported early Sunday.

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The news outlet cited two unidentified sources who said Romney withheld his high-profile endorsement out of respect for Jeb Bush, who ended his campaign late Saturday after a disappointing fourth-place finish in South Carolina primary.

Rubio edged fellow freshman senator Ted Cruz for second place in the Palmetto State, behind Donald TrumpDonald John TrumpDemocrats slide in battle for Senate Trump believes Kushner relationship with Saudi crown prince a liability: report Christine Blasey Ford to be honored by Palo Alto City Council MORE.  Cruz was once considered to have the inside track on South Carolina due to its strong contingent of evangelical voters and Rubio’s strong showing will give his campaign new life headed into the Nevada caucuses.

The Huffington Post reported that Romney, who easily won Nevada in 2008 and 2012, could endorse Rubio before the GOP caucuses on Tuesday.

A Rubio adviser, however, questioned the report, tweeting on Sunday morning that it would be a "bizarre" strategy not to tell the senator's campaign and break the news in The Huffington Post.

--This report was updated at 9:10 a.m.