WASHINGTON — Hillary Rodham Clinton and Barack Obama are essentially tied for the Democratic presidential nomination, according to a USA TODAY/Gallup Poll, the first time that the New York senator hasn't clearly led the field.

The Illinois senator bests Clinton by a single percentage point, 30 percent to 29 percent, if the contest includes former Vice President Al Gore.

Clinton bests Obama by a single point, 37 percent to 36 percent, if it doesn't include Gore.

The survey of 310 Democrats and 160 independents who "lean" Democratic, taken Friday through Sunday, has a margin of error of plus or minus 5 percentage points.

National polls taken the year before an election are "clearly not a prediction of how they're going to end up," cautions political scientist Charles Franklin of the University of Wisconsin. "On the other hand, the national trends are a good indicator of whose messages are beginning to work and whose messages may be falling short."

No other major national poll has shown the Democratic race so close. An ABC News/Washington Post poll taken last Tuesday through Friday gave Clinton a 12-point lead.

Mark Penn, Clinton's chief strategist, calls the USA TODAY poll "an outlier" that is "completely out of sync" with other surveys. He says it is "seriously flawed" for including so many independents unlikely to vote in Democratic primaries.

"National polls are not all that relevant in a process that is largely sequential" through early contests in a series of particular states, says David Axelrod, Obama's chief strategist.

Even so, Axelrod adds, "This poll is consistent with what we see on the ground" as increasingly huge crowds show up for Obama's appearances. He says it also undercuts the argument Clinton strategists have tried to make that "she's an (unconquerable) Juggernaut."

Among Democrats alone, Clinton leads Obama by 5 points, 34 percent-29 percent. That's a significant narrowing from the USA TODAY poll taken in mid-May, when she led by 17 points. Among independents, Obama leads by 9 points, 31 percent-22 percent.

Meanwhile, former New York Mayor Rudy Giuliani leads the Republican field at 32 percent, up three points from the poll in mid-May. Arizona Sen. John McCain was second at 19 percent, down four points.

Former Massachusetts Gov. Mitt Romney edged up four points to third place, at 12 percent, one point above former Tennessee senator and actor Fred Thompson. Thompson formed a testing-the-waters committee last week and could formally enter the race as early as next month.

The survey of 439 Republicans and independents who "lean" Republican has a margin of error of plus or minus 5 points.

The Republicans meet in New Hampshire at 7 ET tonight for their third televised debate, this one sponsored by CNN, WMUR-TV and the New Hampshire Union Leader.