Clinton was preferred by 42 percent of the likely Democrat voters polled, Obama by 33 percent -- a significant increase for him since a similar poll in early December, when he was the choice of 21 percent. Clinton's support remained virtually unchanged during that period.
The overall preference figures mask a pronounced racial divide among Democrat voters: About two-thirds of black respondents said they would vote for Obama, whereas about one-fourth of white respondents said he was their choice.
The poll also found that an increasing number of Democrat voters -- about two-thirds -- say they are certain whom they will vote for, making major swings in preference less likely as the primary season heads into the states with the most delegates at stake.
"Now that Democrats have winnowed down their race to two leading candidates, they are moving toward the candidates they will probably vote for," said Susan Pinkus, the poll's director. "Obama has gained some support, but Clinton has not lost any. The question now is: Where do the remaining voters go?"
Former Sen. John Edwards of North Carolina drew the support of 11 percent of Democrat respondents.
When asked whom they would vote for if their first
In the Republican presidential contest, voter support is scattered among four candidates.
Sen. John McCain of Arizona has inched up to first place, though his lead is within the poll's margin of error. Meanwhile, former New York Mayor Rudy Giuliani, once the clear front-runner nationally in the GOP race, has sagged to fourth place.
The survey found that 22 percent of likely Republican voters preferred McCain, 18 percent backed former Arkansas Gov. Mike Huckabee and 17 percent chose former Massachusetts Gov. Mitt Romney.
Giuliani was supported by 12 percent, down dramatically from 32 percent in an October poll and 23 percent in the December survey. Giuliani decided not to compete actively in the first several contests in the Republican race and has staked his candidacy on a strong showing in Florida's primary Tuesday.
Former Tennessee Sen. Fred Thompson, who ended his candidacy Tuesday after a disappointing showing in Saturday's South Carolina primary, garnered 12 percent, and Rep. Ron Paul of Texas, who has proved surprisingly successful at raising money from a core of supporters, drew 6 percent.
The Republican electorate remains volatile.
Barely more than half of Republican voters -- 52 percent -- were certain that they would stick with their first choice; 41 percent, including about half of Romney supporters, said they might yet change their minds.
"The Republican race is still wide open," Pinkus said. "Republican voters are still trying to figure out who their candidate is ... and most of them aren't very satisfied with their choices."
In hypothetical general election matchups, Clinton and Obama run well ahead of Romney, Huckabee or Giuliani. McCain, by contrast, is the one Republican within striking distance of defeating either of the two Democrats in the general election.
"McCain is the only Republican at this point who would give Clinton or Obama a run for their money," Pinkus said. "Among independents, they are absolutely tied."
The survey, conducted under Pinkus' supervision, was based on interviews Friday through Tuesday of 1,312 registered voters, including 532 who expect to support a Democrat and 337 who expect to support a Republican.
The margin of error was plus or minus 4 percentage points for the Democratic sample and plus or minus 5 points for the GOP group.
For questions asked all registered voters, the margin of error was plus or minus 3 percentage points.
The poll suggests that Clinton has solidified her hold on several core constituencies in her party: Voters older than 65, Catholics and married women all support her candidacy at the level of 50 percent or higher.
Obama, by contrast, draws strong support from blacks and more-affluent voters.