BOSTON — Senator Clinton, Senator Obama, and the rest of the Democratic presidential candidates may be about to welcome a host of party-crashers: New Hampshire’s independents.

Eight years ago, the independents — who are a plurality of the voters in the first-in-the-nation presidential primary — chose in overwhelming numbers to vote in the Republican contest, fueling Senator McCain’s victory.

Next year, according to the University of New Hampshire Survey Center, more than two-thirds of independents plan to vote in the Democratic primary, driven by a desire for change and dissatisfaction with the Iraq war and President Bush.

“They’ll crash the party on the Democratic side and leave the Republicans pretty much alone,” an assistant professor of politics at St. Anselm College in Manchester, Dante Scala, said.

That would shape both parties’ races, Mr. Scala said: To win, Democratic candidates will need to court the independents, while Republicans may focus more on party stalwarts.

The poll director at the University of New Hampshire in Durham, Andy Smith, said the movement of independents to the Democrats might help Mr. Obama, the Illinois senator, more than his New York colleague Mrs. Clinton, who polls show is stronger with the party’s base.

Meanwhile, “for the Republicans, it means your electorate is going to be a solid Republican electorate,” Mr. Smith said. That may benefit candidates such as Mitt Romney, 60, a former Massachusetts governor who is building his appeal around his party’s social- and religious-base voters, rather than Mayor Giuliani, 62, for whom independents are a vital constituency.

The 44% of the state’s electorate who are registered as undeclared have played this role before, helping give upset victories to Democrat Paul Tsongas in 1992 and Republican Patrick Buchanan in 1996, as well as Mr. McCain in 2000.

In last year’s midterm elections, independents helped Democrats oust two incumbent Republican congressmen and, for the first time since 1874, take control of both the governorship and the state Legislature.

In 2008, the battle for independents is leading Democrats to take up issues outside the party’s traditional portfolio, a senior adviser to Senator Biden, Larry Rasky, said. “The political equation for how you address the issues changes,” he said. “It makes it more like a general election.”

An educational-publications editor from Hampton, Mary Lou Heran, 60, may be typical of Granite State independents. Ms. Heran previously cast ballots twice for Mr. Bush, a Republican, and twice for President Clinton, a Democrat. She said she plans to vote for a Democrat in 2008; her reasons — Iraq and what she sees as excessive partisanship in politics — echo the major themes of Mr. Obama’s campaign.

With just two years of experience in Washington, Mr. Obama, 45, is casting himself as an outsider capable of bridging partisan divides, a traditional selling point for independents. “Compared to Hillary Clinton, he’s not seen as involved in politics for as long a period,” Mr. Smith said.

Mr. Obama is also emphasizing his consistent opposition to the war, drawing another contrast with Mrs. Clinton, 59, who voted for the 2002 resolution giving Mr. Bush authority to invade Iraq.

In a May 18 speech in Rye, Mr. Obama reminded his audience that he opposed the war from its outset and said he understands why many Americans are “cynical” about politics, which he said often looked more like “a business instead of a mission.”

Mrs. Clinton, who leads in the polls, is trying to fight the impression that she is a Washington insider anointed by party regulars. “The label of ‘establishment candidate’ is put there by others,” Mrs. Clinton’s New Hampshire director, Nick Clemons, said.

Seeking to appeal to independents, she emphasizes taxes and excessive government spending. In Manchester last month, she announced a 10-point plan to fight waste and corruption, a blueprint that would trim government contractors and curb no-bid contracts.