WASHINGTON (CNN) -- The race for the White House appears to be tightening up in Iowa.

A new poll by Research 2000 finds a three-way battle among the Democratic frontrunners. Of the Democratic caucus voters polled, 28 percent said they would vote for Senator Hillary Clinton. Former Senator John Edwards's support was at 26 percent, with Senator Barack Obama at 22 percent. All the other Democratic candidates were in single digits.

On the Republican side, the new poll also shows an even closer contest. Eighteen percent of Republican caucus voters polled said they'd vote for Senator John McCain. Former New York City Mayor Rudy Giuliani was just one point behind at 17 percent, and former Massachusetts Governor Mitt Romney was at 16 percent. The remaining Republican 2008 hopefuls were all in single digits.

Iowa is a crucial player in the primary calendar. The states caucuses, scheduled for January 14, kick off the presidential contest season.

The Research 2000 poll was taken May 14 through 16 among 600 likely voters. The survey has a margin of error of plus or minus four percentage points.