On the heels of what some participants called their most productive bipartisan meeting to date, a group of GOP and Democratic Senators this week will begin drafting a compromise immigration bill they hope will serve as the leading vehicle for comprehensive reform this Congress.

And while the move may be the clearest sign yet that Senators intend to meet Majority Leader Harry Reid’s (D-Nev.) goal of debating immigration reform the last two weeks of May, some lawmakers worry that major hurdles still stand in the way.

Chief among those concerns is how negotiators can wade through a series of competing proposals, including a new draft plan by the White House, that offer different ways of dealing with the nation’s 12 million illegal immigrants and a variety of takes on how — or whether — to create a temporary guest-worker program.

Senate Judiciary ranking member Arlen Specter (Pa.), one of the leading Republicans in the discussions with Democrats, said Friday that a bipartisan group of Senators is ready to reconcile ideas and “put it on paper” this week. Specter insisted that no particular proposal will be given more weight than the others, but he acknowledged that Senators “will have to be very aggressive” in the coming weeks in what’s going to be a “laborious and time consuming” effort.

“We are well on the road to producing a bill to take up on Sen. Reid’s timetable,” he said.

Senators decided to start crafting legislation following a meeting Thursday that involved White House officials as well as six Republican and four Democratic Senators. The session was the second bipartisan meeting involving the Bush administration in as many days, and such gatherings are expected to become a regular occurrence beginning next week when the Senate returns from recess.

“We’ve made a lot of progress,” said Sen. Mel Martinez (R-Fla.), who has been active in the negotiations. “What we have to do now is take our ideas and draft a bill. That will happen during the break.”

“We have to get started,” added a senior Democratic aide familiar with the immigration talks. “Time is not on the side of those who want to pass comprehensive immigration reform. We have to move as quickly as possible before the pressures of the presidential campaign significantly affect what’s going on on the Hill.”

Martinez acknowledged that some differences among Senators still exist, but he believes Democrats and Republicans are ready to start crafting a “consensus” bill that focuses on what they can agree on. As for any unresolved, controversial pieces, Martinez said some ultimately may have to be decided through amendment votes on the Senate floor.

“I really believe we have a way forward,” Martinez said.

Martinez, Specter, Judiciary Chairman Patrick Leahy (D-Vt.) and Health, Education, Labor and Pensions Chairman Edward Kennedy (D-Mass.) are among the 10 influential Senators who are sitting at the heart of the immigration talks and will be key to crafting any legislation.

Also involved are GOP Sens. Lindsey Graham (S.C.), Jon Kyl (Ariz.), John Cornyn (Texas) and Kay Bailey Hutchison (Texas), as well as Democratic Sens. Ken Salazar (Colo.) and Bob Menendez (N.J.). Homeland Security Secretary Michael Chertoff and Commerce Secretary Carlos Gutierrez also have been engaged, and they helped lead Thursday’s bipartisan meeting.

The six Senate Republicans — whose views on immigration cross the GOP spectrum — have been meeting separately with Chertoff and Gutierrez since January. Those talks served as the impetus for the White House’s latest proposal, which Democrats and interest groups quickly have derided as a step backward from where they were even a year ago.

“Some of these proposals being floated by the White House are very, very problematic,” said Jim Manley, spokesman for Reid. “Sen. Reid is anxious to get a comprehensive bill through the Senate as quickly as possible. However, the only way to get anything done is if this administration stops paying lip service to the right wing and starts negotiating in good faith with Democrats on the Hill.”

The administration’s proposal, which was widely distributed last week, proposes a series of reforms ranging from stiffer border security to temporary visas for illegal immigrants.

Democrats, however, already are balking at several provisions, including requirements that immigrants must return home and pay a $10,000 fine to ultimately achieve citizenship, and that there be no special allowances for immigrant families, unlike last year’s Senate-passed bill.

“The question is how firm they are,” one knowledgeable Democratic Senate staffer said of the White House proposals. “Are the Republicans going to hold on to them? If it’s just a temperature read, then there’s some optimism that we are actually going somewhere.”

Salazar said that even with work under way to finally draft a Senate bill, he would caution against claiming victory just yet. Salazar, like many Democrats, would like the Senate to instead consider an immigration package recently introduced in the House by Reps. Jeff Flake (R-Ariz.) and Luis Gutierrez (D-Ill.) that seeks to bolster border security and provide a path for citizenship for many of the country’s immigrants.

Asked whether Senators would come together successfully on drafting a separate bill, Salazar said: “The jury is still out.”

The House bill largely mirrors a measure offered last year by Kennedy and Sen. John McCain (R-Ariz.), which cleared the Senate by a comfortable margin. Democratic sources suggested they could have a similar bill ready to go again this year.

Senators say they are still hoping to consider immigration legislation in advance of the House, where lawmakers have taken far more strident positions in recent years. The two chambers passed competing bills in the previous Congress but were unable to reconcile their vastly different versions.

Even with the challenges ahead, Graham said late last week that Thursday’s session with the 10 Senators was “one of the most productive bipartisan meetings we’ve had,” and he said he’s as optimistic as he’s ever been about passage of immigration reform. Graham predicted that “within 30 days we can really move.”

“We are trying to get there from here in a way that will make our country proud,” he said. “This is doable.”