Senior Democrats in Washington said Wednesday that they expected Gov. Deval Patrick to name Paul G. Kirk Jr., a former aide and longtime confidant of the late Sen. Edward M. Kennedy, to Kennedy's seat on Thursday. 'This is a very serious and important decision,' Patrick said Wednesday night. 'I expect to make it very, very soon.' But the Democrats in Washington, speaking on the condition of anonymity, said they were certain Kirk was the choice. Earlier in the day, the Massachusetts Legislature gave final approval to a bill that allows Patrick to name an interim successor to Kennedy, who died of brain cancer last month. A person close to the Kennedy family said Wednesday that Kennedy's widow, Victoria Reggie Kennedy, and his sons, Edward M. Kennedy Jr. and Rep. Patrick J. Kennedy, D-R.I., had urged Patrick to appoint Kirk, who worked for Kennedy in the 1970s and later served as chairman of the Democratic National Committee. Kirk, 71, is now chairman of the John F. Kennedy Library Foundation in Boston. Several friends and associates described him as low-key and laconic, a shrewd political strategist who could have run for office himself but decided he preferred a behind-the-scenes role. Democrats in Washington and Massachusetts expressed enthusiasm for his candidacy, saying Kirk was familiar enough with Capitol Hill and Kennedy's priorities to seamlessly pick up where the senator left off. Just before Kennedy died of brain cancer on Aug. 25, he asked the Legislature to change the law and let Patrick appoint a temporary replacement for his seat until a special election could be held. That election is scheduled for Jan. 19. Although Kennedy did not mention it when he made the request, it is clear that Democratic votes will be crucial to passing the contentious health care legislation making its way through Congress. He was a champion of overhauling the health care system, but with his seat empty, Democrats in the Senate are not assured the 60 votes necessary to pass the legislation. Under the state constitution, Patrick has to take the unusual step of declaring the law an emergency to make it effective immediately; most new laws cannot take effect for 90 days. State Republicans said they might try to block an emergency declaration, and indeed late Wednesday, several House Republicans asked Patrick to seek an advisory opinion from the state's Supreme Judicial Court on whether he had the authority to make such a declaration. But William F. Galvin, the secretary of state, said there were no grounds for a legal challenge. 'This procedure goes on all the time,' said Galvin, a Democrat. In addition to Kirk, Patrick was said to have considered Michael S. Dukakis, the former governor and 1988 presidential nominee, and Evelyn Murphy, a former lieutenant governor under Dukakis. Republicans, who have fought the succession bill all along, tried again on Wednesday to stall or quash it. Governors here had the power to fill Senate vacancies until 2004, when the Democratic majority in the Legislature changed the law to require a special election. Democrats worried then that if Sen. John Kerry were elected president, Gov. Mitt Romney, a Republican, would appoint a Republican. The constitution states that to put a new law into effect without delay, a governor must write a letter to the secretary of state declaring that 'the immediate preservation of the public peace, health, safety or convenience' requires it. As an alternative, the Legislature can declare a new law an emergency, but it requires a two-thirds vote. Both chambers tried but failed on Wednesday to muster enough votes for such a declaration. 