The Senate is delving into contentious immigration legislation after a televised prod from President George W. Bush to toughen border security with National Guard troops and find a "rational middle ground" on citizenship for millions of men and women in the United States illegally. The centerpiece of Bush's speech Monday night was his announcement that as many as 6,000 National Guard troops would be dispatched to states along the Mexican border to provide intelligence and surveillance support to civilian Border Patrol agents. The Border Patrol would remain responsible for catching and detaining illegal immigrants. "We do not yet have full control of the border, and I am determined to change that," the president said. Still, Bush insisted, "The United States is not going to militarize the southern border." Democrats responded with a pledge of cooperation and a barbed question for the commander in chief. Bush "has the power to call up the National Guard to patrol the border," said Illinois Sen. Dick Durbin, the second-ranking Democrat. "But does he have the power to lead his own Republican forces in Congress in support of real immigration reform?" Durbin's jab was aimed at anticipated year-end compromise negotiations with House Republicans. But the next move in an election-year struggle belonged to the Senate, where, hours before Bush spoke, debate opened on a bipartisan bill that generally met his specifications. The measure includes tougher border security provisions, a guest worker program and an eventual path to citizenship for nearly all the estimated 12 million immigrants in the country illegally. The bill also includes steps to make sure employers don't hire illegal workers. After months of political bickering, Senate Majority Leader Bill Frist, a Tennessee Republican, and his Democratic counterpart, Sen. Harry Reid of Nevada, agreed the Senate was on track for passage of the bill by the end of May. While much of the advance focus on Bush's speech was on border security - a major issue for conservatives - the president's comments on possible citizenship for illegal immigrants were more explicit than earlier remarks and showed an effort to appeal to moderates and business owners who favor liberalized immigration laws. "Some in this country argue that the solution is to deport every illegal immigrant, and that any proposal short of this amounts to amnesty. I disagree," he said. "It is neither wise nor realistic to round up millions of people, many with deep roots in the United States, and send them across the border. There is a rational middle ground between granting an automatic path to citizenship for every illegal immigrant, and a program of mass deportation." The troops of the National Guard, a military force normally under the control of state governors unless federalized by the president, would serve two-week stints before rotating out of the assignment. Thus, keeping the force level at 6,000 over the course of a year could require up to 156,000 troops. Guard troops are known as "weekend warriors" because they are in the civilian work force and attend occasional meetings. Demands of the Iraq and Afghanistan wars have forced tens of thousands in the National Guard into regular war-fighting duty. Such a new drain on the Guard's forces could become severe new hardships to the regular forces. The White House hopes deployments to the border will begin in early June.