A new Mason-Dixon poll released Tuesday shows the top-three Democratic contenders are in a close race in Iowa, with a surprising 27 percent of voters declaring themselves “undecided” nearly seven months before the nation’s first presidential caucus.  On the Republican side, Arizona Sen. John McCain seems to be tanking, with just 6 percent of Republican caucus-goers saying they’ll vote for him.

MSNBC’s First Read first reported the results Tuesday; details of the polling methodology and results are available to Mason-Dixon subscribers only.

On the Democratic side, 22 percent of caucus-goers said they would support Sen. Hillary Clinton of New York, 21 percent favored former North Carolina Sen. John Edwards and 18 percent preferred Illinois Sen. Barack Obama. New Mexico Gov. Bill Richardson led the second-tier candidates with 6 percent, and Delaware Sen. Joe Biden had 4 percent.

The polling, which included 400 “likely caucus-goers,” has a margin of error of +/- 5 percent. That puts Clinton, Edwards, and Obama into a statistical tie, essentially.

For Republicans, the race seems much clearer. Former Massachusetts Gov. Mitt Romney leads with 25 percent vs. 21 percent for “undecided.”

However, the race gets a bit interesting once you get past Romney, who has consistently been leading in most Iowa polling. Former Tennessee Sen. Fred Thompson registered 17 percent support, while former New York City Mayor Rudy Giuliani stood at 15 percent.  In the second tier, former Arkansas Gov. Mike Huckabee’s 7 percent out-polled both McCain and Kansas Sen. Sam Brownback, who drew 6 percent.

McCain’s collapse into the single-digits seems indicative of a struggling campaign in the state.  It also appears that his decision to skip the Iowa GOP’s August Straw Poll could affect him in the January caucuses.

This is Mason-Dixon’s first poll on the 2008 field in Iowa. The Des Moines Register’s Iowa Poll has consistently shown Edwards with a lead on the Democratic side, with Clinton and Obama battling for second.  On the Republican side, Romney’s lead is consistent with the Iowa Poll results. Thompson’s ascension to second place confirms a national trend that has shown the candidate surging to the top of the Republican ticket.