Lautenberg's Death Adds to Democrats" Legislative Difficulties Christopher Gregory/The New York Times Senator Frank R. Lautenberg, center, made a special effort in May to attend a hearing to support President Obama's nominee to lead the Environmental Protection Agency. WASHINGTON - The death of Senator Frank R. Lautenberg of New Jersey poses new complications for the White House and Democrats on Capitol Hill as they try to push their agenda through a Senate where even a single vote can derail legislation. So crucial was Mr. Lautenberg's reliably liberal vote in a Senate where his party held a 55-to-45 majority that Democratic leaders twice asked him in recent weeks to return to Washington to vote despite his failing health. He died on Monday morning of complications from viral pneumonia. Just last month, Mr. Lautenberg, 89, made a special trip to the Capitol to supply a key vote that saved President Obama's nominee to lead the Environmental Protection Agency from being stalled indefinitely in committee. Republicans were threatening to boycott the vote to deny Democrats a quorum. Mr. Lautenberg agreed to attend to provide the quorum. In one memorable instance in April, his aides helped push him out onto the Senate floor in his wheelchair to cast much-needed "yes" votes for a package of gun-control legislation. Though the bills were ultimately defeated in a Republican-led filibuster, his presence that day served as a reminder for how the majority leader, Senator Harry Reid of Nevada, must scrape for almost every vote. Mr. Reid must scrape even harder now that Mr. Lautenberg's successor is expected to be a Republican. The state's governor, Chris Christie, a Republican, will make the appointment. "The last thing Senator Reid can afford right now is to lose a reliable Democratic vote in a highly partisan Senate," said Jim Manley, a former adviser to the majority leader when the illnesses of Edward M. Kennedy and Robert C. Byrd caused problems for Democrats, who controlled even fewer seats. "He's got a better margin to play with than he did back when Senator Byrd and Senator Kennedy were sick," Mr. Manley added. But the fact of the matter is it's tough to lose a reliable Democratic vote right now. Governor Christie's appointee will arrive in the Senate as the body prepares to tackle some of the most significant domestic policy issues in years. An immigration overhaul is at the top of the Senate agenda and will move to the floor next week. Like any other matter that the Senate takes up these days, the threat of a filibuster all but requires the support of 60 senators, the threshold at which a filibuster can be broken. Though Republicans and Democrats who support significant changes in the country's immigration laws agree that the plan stands a good chance of getting 60 votes, they have been counting every head, leaving nothing to chance. Losing a vote is more troublesome in another issue that has drawn both parties into a bitter feud: presidential nominations. Republicans are threatening to filibuster several of Mr. Obama's cabinet-level nominees, including his choices for E.P.A. administrator and labor secretary. Several other confirmation battles loom. The White House, which has said it is exhausted by Republican efforts to thwart the president's ability to select his own cabinet members and name people to the federal judiciary, is expected to soon nominate three judges at once to serve on a top appellate court, a move that is sure to inflame a situation that is already fraught. Where a single vote could matter the most to Mr. Reid is if he moves ahead with a plan to ram through changes to the Senate's filibuster rules so Republicans are much more limited in how they can block the president's cabinet and judicial nominees. Many Democrats are deeply wary of such a change, known as the "nuclear option," not only because it would alter one of the Senate's most fundamental institutions but also because they could one day find themselves in the minority wishing they had unfettered filibuster power. Mr. Reid has said he believes he needs just 51 votes to change the filibuster rules, though ordinarily rules changes require a two-thirds majority.