WASHINGTON — It took Representative Dan Maffei of New York two tries, $4 million and the retirement of a Republican incumbent to win his House seat last year. After all that, he wants to avoid becoming a one-term wonder because of an unpopular vote on health care. 

“We can’t afford to make enemies,” said Mr. Maffei, a former Congressional aide and a Democratic member of the freshman class that played a role this week in slowing House consideration of a health insurance overhaul, upending plans by Speaker Nancy Pelosi to meet President Obama’s goal of approving a bill before the August recess.

Of 35 first-term Democrats, Mr. Maffei, who represents the Syracuse area, and 25 others occupy suburban, small-town and rural seats they took out of Republican hands, seats Republicans are eager to take back. As a result, junior Democrats want to be sure the emerging health care plan is one they can embrace, particularly after they have already had to cast a difficult vote on climate-change legislation.

“As long as I feel comfortable that I had the opportunity to address issues that I have raised, I am willing to take the tough vote,” said Representative Frank Kratovil Jr., Democrat of Maryland. “What I am not willing to do is follow the lead simply to follow the lead.”

As the House prepared to leave town until after Labor Day, the health bill was taking on some of the ideological hue of House freshmen, many of whom represent districts in Southern and Western states that were previously out of reach of Democrats, far from the urban centers that have long been the party’s base.

But the changes in the health care legislation have already provoked a backlash from the party’s progressive wing, setting up an August struggle not only with Republicans but also among Democrats for the overall direction of the health plan.

Working somewhat as a bloc, and also through other groups like the fiscally conservative Blue Dogs, the freshmen helped not only to postpone a floor vote but also to spur concessions on cost and regional disparities. In addition, in response to freshman demands, party leaders are talking about raising the threshold for any surtaxes that could hit small businesses.

“On issues where we agree, we are not afraid to speak out together and flex our muscles,” said Representative Jared Polis, Democrat of Colorado and an author of a letter to Ms. Pelosi challenging the proposed surtax. “And one of those issues in the context of the health care debate was speaking out against tax increases we saw as too expensive.”

Typically, Congressional freshmen are more seen than heard. But the political precariousness and the sheer size of the freshman class — along with the sophomores elected in 2006, they represent the difference between a Democratic majority and minority — give them more sway than usual.

“They are influential,” said Representative Steny H. Hoyer of Maryland, the majority leader, who helps tend to the care and feeding of the freshmen. “One of the concerns of the freshmen was to give them time, so they are getting time.”

“It gives them an opportunity to explain to their constituents their views of the bill, and secondly, they have more time to figure out what changes, if any, they need to effect,” said Mr. Hoyer, who is urging the freshmen to take the offensive on health care and emphasize the potential benefits of the overhaul.

In an indication of the importance attached to the class, Speaker Pelosi meets weekly with the freshmen for a Wednesday breakfast. Representative Chris Van Hollen of Maryland, the chairman of the Democratic Congressional Campaign Committee, is sending them home for August with a briefing package that offers recommendations on how to sell the health care plan.

To hear Republicans talk, the freshmen are going to need each recommendation.

“I think it’s safe to say that over the August recess, as more Americans learn more about their plan, they’re likely to have a very, very hot summer,” Representative John A. Boehner of Ohio, the House Republican leader, said Thursday.

Republican strategists are planning a monthlong assault on Democrats deemed to be vulnerable, and many freshmen are at the top of the list.

Not only is the first re-election campaign considered the best opportunity to beat an incumbent, but freshmen from formerly Republican districts in states like Virginia, Ohio and Pennsylvania have already been under the gun over the climate-change legislation. Thirteen of the 35 districts now held by Democratic freshmen were carried by the Republican candidate for president in 2008.

Republicans say they do not believe that any modifications to the health plan will be enough to protect Democrats.

“If Democrats think that their situation is improving on health care, then they haven’t been reading the polls,” said Ken Spain, a spokesman for the National Republican Congressional Committee.

Mr. Spain called the emerging bill “political malpractice for a Democrat in even the safest of seats.”

But some of the freshmen say they are willing to take the heat as long as they can have some say in shaping the legislation.

“We wanted to tackle the problems that neither party has had the guts to face for the last few decades, and we are ready to take them on even if that is unpopular,” said Representative Tom Perriello, a Virginia freshman who narrowly defeated a Republican incumbent. “But only if we feel what we are doing is actually solving the problem and not just patching it up.”

Mr. Perriello said August could amount to a monthlong town-hall-style meeting to allow Democrats to explore health care with residents of their districts. He and other freshmen said that as recent survivors of competitive races, they had a good sense of how to gauge public opinion and act accordingly.

“We all just finished very tough races,” said Representative Jim Himes of Connecticut. “Very few of us are in safe Democratic districts. So we listen very, very hard.”