The Obama campaign threw water on Gingrich's swan song by releasing a video of the former speaker of the House of Representatives bitterly maligning his rival for the party's nomination.
Gingrich spent most of his campaign famously angry at Romney, branding him "fundamentally dishonest," a ruthless corporate looter and a reject, among other insults.
In the 90-second video, entitled "Newt Gingrich: Frankly, Not Mitt Romney's Biggest Supporter" — a tip of the hat to one of the politician's pet adverbs — Gingrich calls Romney a liar, labels him a corporate raider who threw thousands out of work, questions why he once needed a Swiss bank account and calls him the most anti-immigrant candidate in the Republican field.
In short, everything that Republican party officials warned Gingrich would happen if he didn't ease up on Romney during their primary season showdowns has apparently come to fruition: the one-time speaker has handed the Obama campaign a treasure trove of ammunition to use against the presumptive nominee.
"As a man who wants to run for president of the United States who can't be honest with the American people, why should we expect him to level about anything if he's president?" Gingrich says in the video
In another clip, with ominous music playing in the background, Gingrich suggests that Romney can't beat Obama.
"The Romney machine can drive down turnout, it can run over opponents with negative ads; it doesn't seem capable of inspiring positive turnout, and the result is, I think, very worrisome if you're thinking about the fall campaign," he says.
And yet Gingrich, 68, now says he will do whatever he can to support Romney in an election battle against Obama.
In his lengthy farewell speech in Virginia on Wednesday afternoon, Gingrich said he's often asked if Romney is conservative enough.
"Compared to Barack Obama?" Gingrich said incredulously.
"This is not a choice between Mitt Romney and Ronald Reagan. This is a choice between Mitt Romney and the most radical, leftist president in American history."
In an interview last month, CBS's Bob Schieffer asked Gingrich how he could endorse Romney after calling him a liar.
"In an ideal world is Mitt Romney the person I'd like to have as president? First of all, that person is Newt Gingrich," Gingrich said.
"(But) there's no question in my mind that Mitt Romney would be a dramatically better president, as would Rick Santorum, than Barack Obama, in terms of the values I hold dear. So it comes down to a question: If you end up with those as your two choices, I would do everything I could do to defeat Barack Obama."
The Romney campaign didn't respond Wednesday to Obama's Gingrich ad. Instead, it released its own video attacking Obama on the economy.
Gingrich's bow-out ends one of the most memorable nomination bids in recent U.S. political history, rife with pie-in-the-sky proposals, snide verbal meltdowns and an apparent hatred for Romney that raged unabated until recently.
After recovering from a campaign in utter disarray last summer to become Romney's chief rival heading into the Iowa caucuses, Gingrich went on the warpath after a barrage of negative advertising against him caused his support in the state to evaporate.
He frequently vowed to bring Romney down, and pledged he'd swipe the nomination away from him in the event of a brokered convention in August in Tampa, even though such a scenario was highly unlikely.
But between the bombs he lobbed at the Romney campaign, Gingrich managed to court ridicule.
He was roundly mocked for his insistence that he'd build an American colony on the moon if elected president. On Wednesday, he acknowledged his wife, Callista, had told him more than 200 times that "the moon colony was probably not my most clever moment in this campaign."
But he suggested time would prove his ambitions correct, adding he intends to continue "cheerfully" advocating for space exploration.
While on the campaign trail, his frequent defence of the sanctity of marriage as he opposed same-sex unions was also the subject of mockery. Gingrich has left two wives for other women; Callista Gingrich is his third spouse and former mistress.
And despite big victories in the South Carolina and Georgia primaries, Gingrich's candidacy failed to ignite. He exits the race with his campaign more than US$4 million in debt.