US Republican presidential candidates have barely one week to win over the Iowa voters who are set to decide the first skirmish of a grueling state-by-state battle for the party\'s nomination. Ahead of the January 3 Iowa caucus, Republicans are overwhelmingly united by a desire to beat President Barack Obama in the November 2012 elections but divided among a crowded field and unsure which challenger to back. The contest does not reliably predict the eventual nominee -- Senator John McCain, the party\'s choice in 2008, came in fourth that year in Iowa -- but is expected to shrink the field as longshot candidates reassess their chances on January 4. Fiery former House speaker Newt Gingrich clings to a fragile lead nationally, according to an average of public opinion polls by the RealClearPolitics.com website that tracks the ebbs and flows of the race. In Iowa, however, Representative Ron Paul leads the pack -- though it is too soon to measure the impact of media reports about racist newsletters that went out under his name decades ago, and which the non-interventionist candidate has disavowed. And former Massachusetts governor Mitt Romney -- the on-again, off-again frontrunner since 2010 -- is seen by the party\'s establishment as the one to bear thanks in part to his vast campaign warchest and major endorsements. But he has never drawn more than 30 percent of Republicans and left cold the core conservatives who typically outnumber their moderate comrades in primaries, amid concerns about past moderate positions on a host of issues. Texas Governor Rick Perry has appealed to Iowa\'s religious conservatives in hopes of righting his once-promising campaign after poor debate performances cast doubt on his ability to take on Obama. Long-shot candidates Representative Michele Bachmann and former senator Rick Santorum have also targeted those voters and visited each of the state\'s 99 counties in a frenzy of pre-Christmas politicking to stay afloat. Former US China envoy Jon Huntsman, however, has largely skipped the heartland state and pinned his hopes on New Hampshire\'s first-in-the-nation primary on January 7, holding some 130 public events there since launching his candidacy. And a popular two-term former governor of New Mexico, Gary Johnson, was expected to shake up the race December 28 by shelving his campaign for the Republican nomination and running instead as a Libertarian. Some public opinion polls suggest that Johnson could siphon enough votes from the eventual Republican candidate to help reelect the embattled Obama, whose hopes are weighed down by the sour US economy and high unemployment. But the Democratic incumbent scored a major victory over his Republican foes in Congress this week in a bitter dispute over the extension of a popular middle-class tax cut, giving his supporters a jolt of enthusiasm. A new Rasmussen Reports poll out Thursday found Romney with fresh momentum, holding at 25 percent, ahead of Paul\'s 20 percent and Gingrich at 17 percent, while the other candidates scored 10 percent or less. But other polls in recent days had given a slight edge to Paul -- a libertarian opposed to foreign aid and overseas military interventions -- indicating the field remains open going into crucial early contests. Last month, Gingrich led the pack in Rasmussen\'s Iowa poll with 32 percent, well ahead of Romney at 19 percent and Paul at 10 percent. Gingrich and Perry suffered a political stumble at the weekend when they failed to gather enough signatures to get on the Republican primary ballot in the state of Virginia. It could spell trouble especially for Gingrich, who has already faced scrutiny for lack of on-the-ground organization in early voting states like Iowa, according to Larry Sabato, head of the Center for Politics at the University of Virginia. \"He was the Virginia front-runner. It also sends a message to voters in Iowa and New Hampshire that his campaign isn\'t serious,\" Sabato told the Richmond Times-Dispatch on Saturday. Recently Romney polled well among the state\'s crucial evangelical Christian voters, with 18 percent, close behind Santorum, at 19 percent. Romney\'s Mormon faith had been seen as a potential obstacle to winning over Christian conservatives.