The protracted battle for the Democratic presidential nomination, which could drag on until the party’s nominating convention in late August, could help Republican congressional candidates in the fall, House Minority Leader John A. Boehner forecast Thursday.

Boehner, R-Ohio, said supporters of either New York Sen. Hillary Rodham Clinton or Illinois Sen. Barack Obama are heading for a big, embittering disappointment when their candidate loses.

“One side is going to be sorely disappointed when the other side wins,’’ Boehner said, adding that some voters who backed the losing Democrat won’t bother to vote in November. That would help the prospective GOP presidential nominee, Sen. John McCain of Arizona, and the party’s congressional candidates, he added.

“The longer this goes on, the more disappointed voters there will be. Frankly, that’s one cause for my optimism,’’ Boehner said at a lunch for reporters organized by the Christian Science Monitor.

Boehner, whose office has begun regular contacts with McCain’s campaign to coordinate policy positions, predicted the Democratic presidential race will last at least until after the last primaries June 3 in Montana and South Dakota.

“You’ve got two candidates who are determined to win. After all the blood, sweat and tears, why would they give up?” he asked. “I wouldn’t give up if I was in their shoes.’’

He predicted House Republicans will pick up seats in November, but stopped short of saying the GOP would regain the majority it lost to the Democrats two years ago.

“I think we will win seats this year, period,’’ he said. “That’s my goal.’’

The National Republican Congressional Committee lags well behind its Democratic counterpart in fund-raising, and far more Republicans than Democrats are retiring or running for other offices.

Boehner cited some successes in recruiting strong candidates for open seats or to take on vulnerable Democrats in GOP-leaning districts. But he admitted recruiting hasn’t gone as well as he expected.

Twenty-five GOP House members are retiring or seeking Senate seats, a factor that in itself will stretch the NRCC’s limited resources, as it tries to keep those seats, rather than going after Democratic freshmen who won in once-GOP districts in 2006.

Jennifer Crider, spokeswoman for the Democratic Congressional Campaign Committee, said the Democratic presidential contest has helped the party by producing big primary election turnouts and record fund-raising. And it could continue to help unless the tone deteriorates sharply.

“If the primary get very nasty and divisive, it won’t be helpful to our candidates,’’ Crider said.

Boehner said he sympathizes with Speaker Nancy Pelosi, D-Calif., chairwoman of the Democratic National Convention, who in recent weeks has been slammed by some Clinton supporters for what they see as a tilt toward Obama. Pelosi rejects that charge out of hand.

“I’m not sure anybody ought to get in the middle of this cat fight. If they do, they’ll get clawed,’’ he said.

Pelosi has been criticized by some Clinton backers for saying the 800 Democratic “superdelegates” should be guided in their presidential choice by the vote for pledged delegates, where Obama has a slight lead. She was still thinking about the issue Thursday.

At the end of a news conference dealing with Iraq, the Speaker stopped reporters before they left the room and said: “You’re not asking me, but I’m telling you. ... It will do great harm to the Democratic Party if it is perceived that the superdelegates overturn the will of the people. That is consistent with a delegate voting his or her conscience.

“I said it when Sen. Clinton was ahead, and I said it when Sen. Obama was ahead — the will of the people must be respected in this election,’’ Pelosi said. She also said it was a big mistake for Democrats to make superdelegates out of the elected officials and party leaders in the early 1980s.