(Reuters) - Democrat Hillary Clinton and Republican Mike Huckabee hold narrow leads on their top rivals two days before the state opens the presidential nominating race, according to a Reuters/C-SPAN/ Zogby poll released on Tuesday.
Clinton, a New York senator, maintained a stable four-point edge over Illinois Sen. Barack Obama , 30 percent to 26 percent, in the Democratic race. Former North Carolina Sen. John Edwards was in third at 25 percent, down one point overnight.
Huckabee , a former Arkansas governor, widened his lead over Romney among Republicans to 29 percent to 25 percent. Romney, a former Massachusetts governor who has been on the attack against Huckabee , slipped two points overnight.
Arizona Sen. John McCain remained a distant third in Iowa's Republican race at 12 percent, with former Tennessee Sen. Fred Thompson gaining two points to climb to fourth at 10 percent.
The poll of 925 likely Democratic caucus-goers and 928 likely Republican caucus-goers was taken Friday through Monday and has a margin of error of 3.3 percentage points for each party.
Iowa is the first test in the state-by-state fight to choose candidates for the November presidential election, and about 7 percent of Democrats and 6 percent of Republicans remain undecided after months of heavy campaigning
Clinton, Obama and Edwards have been battling for the top Democratic spot for months. Clinton, a former first lady who would be the first woman president, leads among women and older voters who are most likely to turn out for a caucus.
But Edwards remains the top second choice of Democrats, a potentially significant factor given the requirement a candidate have 15 percent support in each precinct to be viable or their supporters can switch to another candidate.
Edwards is the second choice of 30 percent of Democrats, with Obama at 22 percent and Clinton at 15 percent.
"Edwards is holding strong among second-choice voters, which could help him on caucus night," said pollster John Zogby . "He also has been cutting into Obama's lead among independents in particular."
Delaware Sen. Joseph Biden and New Mexico Gov. Bill Richardson were at 5 percent each among Democrats, with Delaware Sen. Chris Dodd and Ohio Rep. Dennis Kucinich at 1 percent each.
Among Republicans, Huckabee has solidified his lead over Romney while under attack over his Arkansas record on immigration, taxes and crime.
The rise by Huckabee , a Baptist minister, has been fueled by his support among religious and social conservatives. But Thompson has cut into his backing among Iowans who say they are "born-again" Christians and "very" conservative.
McCain has cut into Romney's support among moderates and independents.
"The battle for third place could be interesting," Zogby said. "Every point McCain gains comes from Romney, and Thompson's gains come from Huckabee . The outcome could determine the winner in Iowa."
Former New York Mayor Rudy Giuliani, who has focused on later states and largely bypassed Iowa, was at 8 percent in the Republican race. Texas Rep. Ron Paul was at 7 percent, and California Rep. Duncan Hunter was at 1 percent.
The rolling tracking poll will continue each day through the Iowa caucus on Thursday. In a rolling poll, the most recent day's results are added while the oldest day's results are dropped in order to track changing momentum.