WASHINGTON - Lawmakers will gather at the White House this week for a working session on immigration legislation, a meeting that has been highly anticipated by Hispanic leaders eager for President Obama to honor his campaign promise to put millions of undocumented workers on a "pathway to citizenship."

But many Democrats are now concluding they might not have the muscle to pass such a controversial measure - at least not immediately, and possibly not until after the 2010 midterm elections.

And even though Obama used a Hispanic prayer breakfast Friday to reiterate his intention to pass some sort of immigration plan during his presidency, this week's gathering demonstrates how the White House and congressional leaders are trying to strike a careful balance.

They are seeking to assuage Hispanic voters, who are a key constituency, while avoiding specific promises on timing and substance, and while trying not to antagonize independent voters who may have a skeptical view of legalization plans.

Obama, for example, slightly recast his citizenship plan promise during Friday's remarks, saying that new legislation should "clarify the status of millions who are here illegally, many who have put down roots."

"For those who wish to become citizens, we should require them to pay a penalty and pay taxes, learn English, go to the back of the line behind those who played by the rules," Obama said.

The biggest obstacle to speedy passage of a citizenship plan, according to interviews with lawmakers and Capitol Hill strategists, is the House. Democrats hold a wide majority there, but at least 40 members represent moderate or conservative swing districts with few Hispanic voters where legalization plans are unpopular and often derided as "amnesty" for lawbreakers.

"This a very, very difficult issue," said Rep. Jason Altmire (D., Pa.), elected in 2006 from a rural western district. "The Democratic Party is doing everything they can to capture this very fast-growing community, and I understand that. But I'm not in that camp."

The White House has downplayed expectations for this week's meeting. According to Hispanic lawmakers who met with Obama this spring, he had promised to host a summit with lawmakers and advocacy groups, just as he did with health-care leaders when he kicked off the debate on that issue. Instead, the immigration event will be small and private and will include only House and Senate members involved in the immigration debate.

Moreover, the White House is careful to point out that Obama wants to merely begin the debate this year. He is not promising that a plan will be passed in 2009, though in his campaign he said he would make the issue "a top priority in my first year as president."

Since then, Obama has made clear he has two legislative goals for the year - a hugely expensive health-care plan and a controversial global-warming bill - that already are putting many swing-district Democrats in a political bind.

Still, some vocal Hispanic activists, led by Rep. Luis Gutierrez (D., Ill.), have pledged to keep applying pressure.

Emma Lozano, an activist married to one of Gutierrez's congressional staffers, is organizing a demonstration to take place outside the White House during the immigration meeting, and Gutierrez has promised to tell demonstrators immediately following the meeting what the president said or did not say.

"We need to hear, 'This is what we're for and this is the timeline,' " said Gutierrez.

As recently as 2008, some party leaders, including now-White House chief of staff Rahm Emanuel, were advising Democratic candidates in swing districts to steer clear of immigration, or take a hard line by calling for stricter border enforcement and regulation of employers.

Surveys in swing districts presented to Democratic candidates by pollster Stanley Greenberg portrayed support for legalization as a political loser. But Obama's success with Hispanics last year has prompted some Democrats to find a rhetorical middle ground, as has the widespread belief that the party's political dominance relies in part on solidifying its standing with the fast-growing Hispanic bloc.

Greenberg produced new swing-district polling last summer to counter his earlier surveys - this time reporting that "a policy and message that focuses on requiring illegal immigrants to become legal expands the Democratic advantage on the immigration issue."

One senior Democrat intent on acting this year is Senate Majority Leader Harry Reid (D., Nev.), who faces a reelection campaign in his home state, where Hispanics are a fast-growing constituency. He has pledged to push legislation in the fall.

But prior efforts have failed in the Senate. And with the measure's longstanding champions, Sens. Edward M. Kennedy (D., Mass.) and John McCain (R., Ariz.), no longer taking the lead, strategists say that success is possible only if Obama steps in.