ARLINGTON, Va.-Newt Gingrich  officially ended his campaign for the Republican presidential nomination Wednesday.

"Today I'm suspending the campaign, but suspending the campaign doesn't mean suspending citizenship," Gingrich said to a roomful of reporters who had waited a week for Gingrich to officially announce it. "Callista and I are committed to being active citizens."

Gingrich said he still wanted to speak on a number of conservative issues and campaign for Republican candidates, including  presumed nominee Mitt Romney.

Gingrich did not officially endorse Romney today, but Gingrich spokesman R.C. Hammond told ABC News Tuesday that a Romney endorsement would come in the next few weeks.

"I'm asked sometimes, 'Is Mitt Romney conservative enough?' And my answer's simple: Compared to Barack Obama? You know this is not a choice between Mitt Romney and Ronald Reagan," Gingrich said. "This is a choice between Mitt Romney and the most radical, leftist president in American history."

Upon his exit, Gingrich thanked Sheldon and Miriam Adelson, the billionaire casino owner couple who kept his campaign alive. The Adelson family donated more than $20 million to Gingrich's super PAC over the duration of  his campaign.

"It would be impossible for me to be here and thank everybody without mentioning Sheldon and Miriam Adelson, who singlehandedly came pretty close to matching Romney's super PAC," Gingrich said.

Gingrich also thanked two former candidates who endorsed him, Rick Perry and Herman Cain. Gingrich said Perry told him "'I'm with you until the cows come home.'" He  thanked Cain for his support on the trail. Gingrich also apologized to the state of South Carolina, the only state other than Georgia that Gingrich won in the primary.

"I will have broken their tradition of always picking the nominee," Gingrich said.

Gingrich also took the opportunity to poke some fun at himself, saying his mention of the moon colony was "not my most clever comment in this campaign," but added  he provided material for "Saturday Night Live."

"My wife has pointed out to me 219 times that 'moon colony' was probably not my most clever moment," Gingrich said. "But the underlying point is real. If we have to be the leading country in the world, we have to  be the leading country in space."

Gingrich is expected to go back on the trail to campaign for Romney in the coming weeks, when his endorsement is likely to come.