Newt Gingrich all but withdrew from the Republican presidential race on Sunday, acknowledging the obvious: He won't win the GOP nod, and Mitt Romney is "far and away the most likely" nominee. Gingrich also pledged to work with the GOP nominee "to help defeat Obama any way I could." But after months of harsh, personal criticism, "the question isn't really what Gingrich can do to help Romney," says Brian Montopoli at CBS News. "It's whether he'll be able to even partially undo the damage he's already inflicted on Romney." Newt may say that his attacks on Romney are water under the bridge, but Democrats are sure to use the former House speaker's "significant gift to the Obama campaign" in ads undermining the GOP nominee. Here, six of Gingrich's most potentially damaging Romney verbal smackdowns:

1. Romney is a liar

The setup: Before the Iowa caucuses, Romney helped erase Gingrich's double-digit lead in the polls with a series of devastating negative ads, which Gingrich branded as dishonest.

The quotes: "I've been Romney-boated.... Somebody who will lie to you to get to be president, will lie to you when they are president." (Jan. 1, in Waterloo, Iowa) "[Romney's] not telling the American people the truth. It's just like this pretense that he's a conservative.... I just think he ought to be honest with the American people and try to win as the real Mitt Romney, not try to invent a poll-driven, consultant-guided version that goes around with talking points." (Jan. 3, on CBS's The Early Show)

2. Romney is insincere

The setup: After Iowa, Romney and Gingrich sparred at the second of two debates before New Hampshire's primary. Romney started the debate talking about how civic-mindedness is the prime motivation for his political and business career, and the reason he didn't run for a second term as Massachusetts governor: "I didn't go there to begin a political career, running time and time again. Run again? That would be about me."

The quote: "Can we drop a little bit of the pious baloney? The fact is, you ran in '94 and lost. That's why you weren't serving in the Senate with Rick Santorum. The fact is, you had a very bad re-election rating, you dropped out of office, you had been out of state for something like 200 days preparing to run for president.... So this idea that suddenly citizenship showed up in your mind — just level with the American people: You've been running for — at least since the 1990's." (Jan. 8, at GOP debate in Concord, N.H.)

3. Romney is a job-killing corporate raider

The setup: Right before the New Hampshire primary, a pro-Gingrich super PAC got a $5 million windfall from casino magnate Sheldon Adelson — and used it to air a 30-minute documentary in South Carolina trashing Romney and the private equity firm he built, Bain Capital. The "vicious" documentary, When Mitt Romney Came to Town, moved the GOP primary from "the 'negative' phase to the 'from Hell's heart, I stab at thee' phase," says James Poniewozik at TIME.

The quote: "Is capitalism really about the ability of a handful of rich people to manipulate the lives of thousands of other people and walk off with the money, or is that somehow a little bit of a flawed system?... I do draw a distinction between looting a company, leaving behind broken families and broken neighborhoods, and leaving behind a factory that should be there." (Jan. 9, in Manchester, N.H.)

4. Romney is an out-of-touch rich guy

The setup: After winning South Carolina, Gingrich's momentum was halted (for good) with another lopsided ad war from Romney and his allies. At the same time, after weeks of prodding, Romney released his 2010 and 2011 tax returns. "Obama's campaign badly wants swing voters to see Romney as a symbol of a protected, super-affluent class that has grown even richer while working Americans have suffered," says Steve Kornacki at Salon. "Gingrich's words could be useful in making this case."

The quote: "We're not going to beat Barack Obama with some guy who has Swiss bank accounts, Cayman Island accounts, owns shares of Goldman Sachs while it forecloses on Florida and is himself a stockholder in Fannie Mae and Freddie Mac while he tries to think the rest of us are too stupid to put the dots together to understand what this is all about.... People matter more than Wall Street." (Jan. 26, in Mount Dora, Fla.)

5. Romney has "no principles"

The setup: Romney adviser Eric Fehrnstrom stepped on his boss' big Illinois in March win by telling CNN that Romney can basically start the general election with a clean ideological slate, "almost like an Etch A Sketch," to woo centrist and independent voters.

The quotes: "I took very seriously the Etch A Sketch comments of his director of communications. ... You can't run a campaign with no principles and win the United States. You are not going to beat Barack Obama by being clever." (April 2, in Frederick, Md.)

6. Romney backs "radical... right-wing social engineering"

The setup: House GOP budget wonk Paul Ryan released his controversial spending blueprint on March 20, with the same general outlines as the Medicare-voucherizing document he released in 2011. Criticizing the budget and chiding Romney for supporting it, Obama recently reminded voters that the "renowned liberal Newt Gingrich first called the original version of the budget 'radical,' and said it would contribute to 'right-wing social engineering.'"

The quote: "I'm against ObamaCare, which is imposing radical change, and I would be against a conservative imposing radical change.... I don't think right-wing social engineering is any more desirable than left-wing social engineering... I don't think imposing radical change from the right or the left is a very good way for a free society to operate." (May 15, 2011, on NBC's Meet the Press)

Sources: ABC, Boston Globe, BusinessWeek, CBS (2) (3) (4), NPR, Salon (2), TIME, Washington Post