By John Whitesides
DES MOINES, Iowa (Reuters) - The budding rivalry between surging Republican Newt Gingrich and former frontrunner Mitt Romney will take center stage on Saturday in the first of two presidential debates in Iowa over the next five days.
Gingrich, Romney and four other White House contenders will make their case to voters in a race polls show is still up for grabs less than a month before Iowa kicks off the state-by-state Republican nominating contest.
The debate will be broadcast nationally on ABC at 9 p.m. EST from Drake University in Des Moines. Another Republican debate will be held on Thursday in Sioux City, Iowa.
"These are the last chances for the candidates to really have an impact on the race before the voting starts," said Republican strategist Matt Mackowiak. "It's a huge opportunity."
It is the first debate since Gingrich, a former House of Representatives speaker, roared past Romney to take a big lead in polls in the Republican battle to pick a 2012 challenger to President Barack Obama.
Romney, the former Massachusetts governor and once the presumed nominee, has responded by cranking up his criticism of Gingrich's record and drawing contrasts between his own background as a businessman and Gingrich's experience in Washington.
"The governor is going to defend his record and point out the differences with the others on the stage," a senior Romney adviser said of his debate plans.
Gingrich has rarely been a target in previous debates and has refrained from attacking his fellow Republicans, but that could change on Saturday night.
His campaign organized a conference call on Friday to respond to Romney's most recent attacks, calling them a sign of panic and desperation and promising to fight back.
"No campaign just lays down and allows an opponent to stab you in the heart," said former Congressman Greg Ganske, a Gingrich supporter. "It deserves a response."
The debate also could be one of the last chances for other candidates to make an impression. Former U.S. Senator Rick Santorum, Texas Governor Rick Perry, and U.S. Representatives Michele Bachmann and Ron Paul also will take part.
All four have been aggressive in criticizing rivals in past debates, and they will have more time to talk as the stage will be less crowded than in earlier debates.
Businessman Herman Cain withdrew from the race a week ago after charges he had a 13-year extramarital affair, and former Utah Governor Jon Huntsman will not participate.
The race has seen a series of Republican White House contenders rising to the top of the pack, only to fall back in popularity after campaign missteps.
The two debates this week could play a role in determining if Gingrich follows in their footsteps.
"Now Gingrich is the frontrunner," Mackowiak said. "It's time to shine or wilt."
(Editing by Eric Walsh)