By Mark Felsenthal WASHINGTON (Reuters) - U.S. lawmakers clashed on Sunday over whether to let some illegal immigrants stay in the United States and work for citizenship, suggesting compromise may elude Congress on a politically sensitive issue. "There's a chasm between the House and the Senate," Illinois Democratic Sen. Richard Durbin said on CBS's "Face the Nation." The Senate is debating a bill that would tighten security along the Mexican border, create a temporary "guest-worker" program, and could create a process for some of the estimated 11 million illegal immigrants in the United States to become citizens. The immigration issue has taken on heightened importance ahead of November congressional elections and poses a dilemma for U.S. President George W. Bush, who wants Congress to approve a guest-worker program despite strong opposition from within his own Republican Party. Durbin and other Senate Democrats said they oppose the approach adopted by the House of Representatives, which in December passed a measure that would define illegal aliens as felons and would build a 700-mile fence along the border with Mexico to keep illegal immigrants out. "The House approach is unacceptable," Durbin said. Meanwhile, prominent Republican lawmakers said the guest-worker program is at odds with the immediate goals of legislation beefing up border security. "If we don't firm up the border, the guest-worker program is going to encourage more people to enter the country illegally," said Rep. James Sensenbrenner, the Wisconsin Republican chairman of the House Judiciary Committee, speaking on the same program as Durbin. A Republican Senator and potential presidential candidate in 2008, George Allen of Virginia, broke ranks with President Bush, saying the legislation should focus on border security and put off debate on a guest-worker program. Bush has backed allowing illegal workers to have temporary legal status while performing jobs Americans are unwilling to do. He favors a comprehensive immigration bill, while some Republicans prefer a limited bill addressing only border security. "It may be several years down the road or months down the road we can get a consensus on how you handle a good temporary-worker system," Allen said on ABC's This Week. "I don't think we ought to be passing anything that rewards illegal behavior or amnesty," he said. Sensenbrenner acknowledged that division in Congress over the guest-worker program poses a major obstacle to a compromise on immigration legislation. "This will be tough, and it's the toughest thing that I've done in 37 years in elective public office," he said. The House bill caused an uproar in the Hispanic community and has drawn opposition from groups as diverse as the U.S. Chamber of Commerce and the Catholic Church. On Saturday in New York, thousands of immigrants and their supporters chanted, blew whistles and waved flags from Latin American countries as they marched across the Brooklyn Bridge in the latest of several protests in U.S. cities against the legislation.