House Democratic leaders began a series of immigration “listening” sessions Wednesday, but they may not have a bill ready before September — a timetable some backers say could kill the effort.

House Democrats say Speaker Nancy Pelosi is holding off on legislative action until the Senate passes its bill (S 1639) — if it does — because she does not want to force a vote on a politically tough proposal if it will not become law.

“We had always hoped to have something done before the August recess,” said Rep. Zoe Lofgren, chairwoman of the Judiciary subcommittee that deals with immigration. “The longer the Senate takes, the more challenging that becomes.”

Lofgren, D-Calif., has yet to unveil a draft bill, House leaders are holding “listening” sessions this week and next with rank-and-file lawmakers rather than committee markups, and Pelosi has set a high bar for House consideration, including evidence of up to 70 Republican votes.

All of that raises doubts both within and outside Pelosi’s caucus about the feasibility of a July timeline for House action.

After the August recess, many caution, the proximity of the 2008 election year — and the hot rhetoric of the presidential campaign — will add to the difficulty of clearing a major policy change on such an emotional issue.

“If it hasn’t passed both houses by the end of July, it is toast,” predicted Rep. Jeff Flake, R-Ariz., who wrote an immigration bill with Rep. Luis V. Gutierrez, D-Ill.

Democrats insist that Pelosi wants to pass an immigration bill, but Republicans survey the same landscape and see plenty of reasons why the California Democrat might hope it gets buried.

“If Pelosi takes up the Senate bill and moves it further to the left, it will convert our biggest potential liability into a motivating force for our base,” said Florida Rep. Adam H. Putnam, the Republican Conference chairman.

Democratic divisions have, to this point, been obscured by Republican infighting. But when the House considers an immigration bill, the majority of House Democrats are likely to be at odds with swing-district colleagues, particularly freshmen whose 2006 electoral victories gave Democrats their first House majority in a dozen years.

“I oppose the bill,” said Democratic Rep. Jason Altmire, a freshman who unseated Republican Melissa A. Hart in western Pennsylvania’s socially conservative 4th District.

Being able to cast a vote against an immigration bill may not be a strong enough political shield for swing-district Democrats worried about 2008. “I wouldn’t want this to be viewed as a Democratic initiative,” Altmire said.

In other legislative debates, Pelosi has made clear that she has little interest in bringing bills to the floor that divide Democrats.

Nonetheless, Democratic leaders are holding six “listening” sessions in which House Majority Whip James E. Clyburn, D-S.C., and Xavier Becerra, D-Calif., are discussing legislative options.

Some Democrats say that after years of debating immigration, the listening sessions are unlikely to yield many new votes.

“We don’t need no stinkin’ listening sessions,” said one veteran lawmaker, paraphrasing from the film “Blazing Saddles.”

Democratic leaders remain reluctant to discuss their plans for an immigration bill.

“Let’s see what the Senate does and then we’ll decide what we’re going to do,” Democratic Caucus Chairman Rahm Emanuel, D-Ill., said.

Estimates of the number of Republicans who would vote for a measure such as the one currently being considered by the Senate, including Emanuel’s own, range from a dozen to three dozen, far less than the 50 to 70 that Democratic leaders say they want to see before they will move a bill.

That threshold, Democrats say, would not only provide a safe margin for victory but give the policy the politically important imprimatur of broad bipartisanship.

The stalled Senate bill is slated for resurrection later this week, but success is far from assured.

If the Senate does pass its version, supporters in both parties and the business community and among Hispanic advocacy groups will pressure Pelosi to act quickly, which may explain the effort to build support through “listening” sessions before a bill has been unveiled.

Keith Ellison, D-Minn., a freshman member of the Judiciary Committee who has been involved in immigration discussions with Democratic leaders, said he would be “pleasantly surprised” if the House considered a bill before the monthlong August recess.

“We may be running out of time before August,” said Loretta Sanchez, D-Calif., chairwoman of the Homeland Security Subcommittee on Border, Maritime and Global Counterterrorism.

The Homeland Security Committee is one of at least four panels that would have jurisdiction over pieces of the bill currently being considered in the Senate.

Judiciary Committee Chairman John Conyers Jr., D-Mich., said the subcommittee that handles immigration for his panel would mark up a bill one week and the full committee would mark it up the next week.

Education and Labor Committee Chairman George Miller, D-Calif., said his panel might consider the legislation, too, but that he could also give up his right to mark up the bill and instead negotiate behind the scenes to expedite consideration.

Ways and Means Chairman Charles B. Rangel, D-N.Y., also is heavily involved in intraparty immigration talks.

Mike Thompson, D-Calif., who is close to Pelosi, said he is “cautiously optimistic” that bills could move through both chambers before the August recess. “If it goes beyond that,” he said, there might not be an immigration measure “for five or six years.”

House Democrats say that legislation could move quickly if a broad, bipartisan consensus can be reached.

But that seems unlikely.

Many conservatives in both parties insist that enhanced border enforcement should be the primary focus of Congress, and oppose the Senate’s “Z” visas, which would make some illegal immigrants legal.

Liberal Democrats have embraced the bill written by Gutierrez and Flake that would allow illegal immigrants to adjust their status as long as they leave the country and come back in through any port of entry. It also would offer a more lenient path to citizenship.

They oppose Senate efforts to give visa preference to highly skilled and educated workers rather than to family members of people already living in America.

Lawmakers say that instead of moving the Gutierrez-Flake bill, Pelosi wants a bill that combines some of its principles and some parts of the Senate bill.

Despite the long odds, Emanuel has been meeting with Flake, who is trying to recruit support among Republicans.

“I think the Democrats understand better than we did that when you’re in the majority, you’re expected to do something and not just punt,” Flake said.

“If the Democrats want to do it, they can do it.”