Paul G. Kirk Jr. , a longtime Democratic Party insider and associate of the late Massachusetts Sen. Edward M. Kennedy , was appointed Thursday by Gov. Deval Patrick as Kennedy’s interim replacement.

Kirk will be sworn in at 3:15 p.m. Friday and will serve until the Jan. 19 special election that will fill out the remainder of Kennedy’s unexpired term, which runs through the 2012 election. As expected said he will not be a candidate for that special election.

Kirk, a senior aide to Kennedy throughout the 1970s, was the choice of Kennedy’s immediate family. He has been serving as chairman of the board of the John F. Kennedy Library Foundation — named for the late president who was the senator’s brother — and officiated at a memorial service held at the library following the senator’s death of cancer Aug. 25.

In addition to his close ties to the Kennedys, Kirk was Democratic National Committee chairman from 1985 to 1989, and is the longtime co-chairman of the Commission on Presidential Debates.

He is considered a party insider with strong knowledge of the legislative process. Kirk pledged to be “a voice and a vote for [Kennedy’s] causes and his constituents in the Senate that he loved.” And he said he would maintain the senator’s existing staff.

The selection of Kirk by the Democratic governor fulfills a request by Kennedy, sent by letter days before he died, to change state law to allow for an interim replacement. Under existing law, the seat would have remained vacant until the special election. When Kirk takes office, he will restore the number of senators who organize with the Democrats to 60, the threshold for the theoretical “filibuster-proof” majority.

In tapping Kirk, Patrick passed over former Gov. Michael S. Dukakis, the 1988 Democratic presidential nominee, disappointing a strong contingent of his supporters in the state.

“The Dukakis people wanted this as a final thank you to Michael for his service,” said Michael Goldman, a political consultant at Government Insight Group, who worked in Dukakis’ administration.

Women’s groups had taken another tack, pushing for Patrick to tap a woman senator, perhaps former Lt. Gov Evelyn Murphy.

Meanwhile, Republicans and good government advocates are already raising questions about Kirk’s corporate ties, including his position on the Board of Directors at The Hartford Financial Services Group. The Hartford provides retiree health insurance plans for employers, giving it a stake in the current health care overhaul debate.

Kirk returned to Massachusetts in the 1990s after his turn in national politics and has focused on legal work and serving on corporate and philanthropic boards since.

He also serves on the boards of Rayonier, Inc., a timber and real estate firm, and Cedar Shopping Centers. He is now retired as a lawyer, but was a longtime partner of the Boston-based firm Sullivan & Worcester, which has a federal and state-level lobbying practice.

Kirk has kept a low political profile in Massachusetts, said Goldman. “Paul’s always been somebody who in the political community had great weight and great gravitas, but among average voters this is the first time they’ll have heard his name,” he said.