Des Moines, Iowa (CNN) -- It didn't take long for Newt Gingrich and Mitt Romney to trade barbs at Saturday night's Republican presidential debate.
Romney, the former Massachusetts governor who has lost the top spot in national polls, was quick to point out where he and the former House speaker differ on the issues.
"We could start with this idea to have a lunar colony that would mine minerals from the moon," said Romney, referring to a Gingrich proposal to mine for precious minerals on the moon.
Romney also highlighted his differences with Gingrich over altering child labor laws and lowering the capital gains tax for most Americans. Romney, as he has in the past, characterized Gingrich as a career politician.
Gingrich calmly responded, saying "the only reason you didn't become a career politician is you lost to Teddy Kennedy in 1994. It's a bit much. You'd have been a 17-year career politician by now if you'd won."
Rep. Ron Paul of Texas, who also has criticized Gingrich over the past few weeks, said that the former House speaker is "taking positions that are not conservative."
The debate, hosted by ABC News, the Des Moines Register and the Republican Party of Iowa, is being held at Drake University in Des Moines.
A lot's changed for the former House speaker since the last GOP debate, a CNN showdown in Washington just before Thanksgiving.
Since then, Gingrich's numbers have soared. He's the front-runner in latest national polling and in new CNN/Time ORC International surveys in Iowa, South Carolina and Florida, three of first four states that kick off the presidential primary and caucus calendar. The CNN poll also indicates that he's closing the gap with Romney in New Hampshire, which holds the second contest.
Gingrich's campaign was left for dead by many in May and June, after a number of controversies spurred some of his top advisers and staffers to quit, and that left the campaign coffers in the red.
But the former House speaker has performed well in the 11 major GOP presidential debates this year, often acting as the elder statesman while many of his rivals for the nomination attacked each other. He also won over the audiences by repeatedly criticizing the moderators' questions.
Thanks to his debate performances and the fading poll numbers of contenders such as Texas Gov. Rick Perry, Bachmann and businessman Herman Cain, who suspended his campaign last weekend, Gingrich's numbers have skyrocketed.
The timing is also crucial. The debate, which will be followed by a second Iowa showdown five days later, comes less than four weeks before Iowa's January 3 caucuses.
CNN's Paul Steinhauser, Rachel Streitfeld, Shawna Shepherd and Adam Aigner-Treworgy contributed to this report.