The Democrats lack enough votes within their party and must overcome lingering Republican disdain for what some consider "amnesty" for some people in the U.S. illegally, as well as union opposition.
The difficulty of their predicament is showing.
The bill's Senate sponsors couldn't agree and gave up their alliance on joint legislation, while its House sponsors introduced their own version, knowing its prospects are heavily dependent on Bush.
"We need to get a bill and he needs to start twisting a few arms," said the Senate majority leader, Harry Reid, D-Nev.
Although Bush supported last year's Senate bill, he also signed a House Republican bill calling for a 700-mile fence on the U.S.-Mexican border. And he did little to persuade Republicans to negotiate an immigration bill.
Originally, Sens. Edward M. Kennedy, D-Mass., and John McCain, R-Ariz., planned to introduce a sweeping immigration reform bill with their House partners, Reps. Luis Gutierrez, D-Ill. an Jeff Flake, R-Ariz.
McCain's enthusiasm withered as he faced a mounting challenge from former New York Mayor Rudy Giuliani and conservatives on the presidential campaign trail unwilling to accept anything less than deportation of illegal immigrants.
In response, McCain broke with Kennedy and said he is looking at proposals that could pass, including one requiring illegal immigrants to self deport and apply for jobs in the U.S. through private employment centers set up in certain countries.
"Both parties are clear where they need to be on a bipartisan bill and a realistic bill, but the politics of getting there is not easy and the clock is running," said Deborah Meyers, senior policy analyst with the Migration Policy Institute.
Gutierrez and Flake have gone forward with the bill they began drafting with Kennedy and McCain, and have made it the starting point for debate in the House.
They hope to lure Republicans and provide some cover for Democrats elected on tough immigration platforms with a provision requiring illegal immigrants to leave the country at some point during a six-year period and return legally.
Even so, "there aren't enough Democrats in the House to pass comprehensive immigration reform. There aren't enough senators, Democratic senators ... to pass it," Gutierrez said.
On the House side, Several Democratic freshmen campaigned against so-called amnesty to help their party win control of Congress.
Among them was Rep. Nick Lampson, D-Texas, who won the conservative Republican district once held by former Majority Leader Tom DeLay.
"He would not support a bill that has a road to legal residency for illegal and undocumented workers who are already here," said Lampson spokesman Bobby Zafarnia.
During his recent visit to Mexico, Bush pledged to intensify his push for a comprehensive immigration bill.
He said he would work to reject "protectionist sentiments" that are bogging down the debate, and named Kennedy as his key ally in the effort.
He dispatched Homeland Security Secretary Michael Chertoff, Commerce Secretary Carlos Gutierrez and some White House officials to meet with a group of Republican senators on immigration over several days.
The meetings have included Sens. Jon Kyl, R-Ariz, and John Cornyn, R-Texas. Both voted against last year's Senate bill and wrote their own bill that would have required illegal immigrants to return home, even for a short period, before getting on a path to citizenship.
Cornyn and Kyl also are critical of proposals to allow future guestworkers a shot at citizenship too, another feature of last year's Senate bill.