Gingrich, a former U.S. House speaker, was expected to announce in Washington he was withdrawing from the race and throwing his support to Romney. The two spoke by phone last week, a Gingrich spokesman said.
Gingrich offered his services in any way the Romney campaign would like to use him, spokesman R.C. Hammond said.
There had been reports Gingrich would announce Tuesday he would end his campaign but CNN, citing a source close to the 69-year-old politician, reported Gingrich would end his bid Wednesday.
Gingrich faced increasing pressure to step aside in recent months as he and former U.S. Sen. Rick Santorum of Pennsylvania split the conservative vote. Calls from the Republican establishment appealed to Gingrich to drop out and make the campaign a two-man race between Santorum and Romney, a former Massachusetts governor.
Santorum dropped out of the race April 10.
Gingrich would end his presidential run with millions in campaign debt and what many political observers say is a bruised reputation.
But "I'm not going to worry about Newt Gingrich," Fred Barnes, conservative commentator and executive editor of The Weekly Standard magazine, told the Atlanta Journal-Constitution. "He's going to find his way. He always has."
Kerwin Swint, a political science professor at Kennesaw State University, part of the University System of Georgia, told the newspaper Gingrich "could fade as a figure unless he's able to maintain his presence in the campaign, either on CNN or Romney campaign appearances."
"I think he's got a lot riding on the next several months," Swint said.