By Molly K. Hooper, CQ Staff

The election of Joseph R. Biden Jr. as vice president, should it occur, would set off a potentially complex pair of political chain reactions: in the Senate, where he’s served for 36 years, and in his home state of Delaware.

Biden, first elected in 1972, is the senator with the third-longest current Democratic tenure, surpassed only by a pair of Democrats, Edward M. Kennedy of Massachusetts, who was first elected in 1962, and Robert C. Byrd of West Virginia, whose first win was in 1958.

The biggest practical impact on the Senate of a possible Biden departure would be his relinquishing the top spots on two of the most influential committees: Foreign Relations, which he has chaired since his party took back control of the Senate last year, and Judiciary, which he chaired from 1987 through 1994 and where he still ranks second in seniority. That could set off a seniority-based chain reaction at the top of half a dozen Senate panels.

There is a reliable assumption that Democrats will retain control of the Senate in this fall’s election. So if the Obama-Biden ticket wins the national race, requiring Biden to resign his Senate seat, the first dibs on the Foreign Relations gavel would go to Christopher J. Dodd of Connecticut. Dodd, who began his adult life as a Peace Corps volunteer and mounted his own short-lived bid for the Democratic presidential nomination this year, has an intense interest in foreign policy.

But to take over at Foreign Relations, Dodd would have to give up the chairmanship of the Banking, Housing and Urban Affairs Committee, which could have important roles to play next year in implementing the next president’s domestic and economic policies — especially if the mortgage crisis persists. One senior Foreign Relations committee aide, who declined to be identified discussing internal maneuvers, predicted that Dodd would not walk away from a position in which he could have enhanced influence over economic and housing issues should Obama win the White House.

Dodd’s office released a statement Sunday noting that the chairman remains focused on his Banking panel duties, enumerating the list of high-profile issues the panel must address in the coming months: “The committee will be overseeing the implementation of the landmark housing bill that passed in July, and will continue its work on other issues such as reforming the practices of credit card companies, improving the regulation of financial institutions, strengthening U.S. financial markets, modernizing our nation’s infrastructure, and advancing legislation that has already passed the committee including Iran sanctions legislation, a bill to address currency manipulation, and flood insurance reform.”

If Dodd chose to stay at the helm of Banking, John Kerry of Massachusetts, the party’s 2004 presidential nominee, would be able to claim the Foreign Affairs chairmanship. That in turn would open up the chairmanship of the Small Business Committee, perhaps to Iowa’s Tom Harkin , who currently chairs the Agriculture Committee and is a heavy favorite to win re-election in his November Senate contest.

If Dodd moves out of the Banking chair, next in line there would be Tim Johnson of South Dakota, who also is a solid favorite to win a new term in November. Should Johnson take over as chairman of such an influential committee, it would mark a remarkable comeback for a lawmaker who nearly died of a brain hemorrhage two years earlier.

Senate Democratic caucus rules allow members to chair one full committee, although a senator may chair one committee and the subcommittee of another. Because Delaware is one of the states that permits people to run for two federal offices at once, Biden plans to maintain his campaign for his seventh Senate term this fall. He is the prohibitive favorite to defeat the GOP nominee Christine O’Donnell, a marketing consultant and political commentator.

If he’s elected vice president as well on Nov. 4, he would have until inauguration day, Jan. 20, to resign his Senate seat. The governor would then appoint a successor who would serve until a special election in November 2010.

The incumbent governor, Democrat Ruth Ann Minner , is term-limited and so would be in position to fill the seat only until she retires at the end of the year. After that the Senate vacancy would be filled by her successor, who is highly likely to be the winner of the Sept. 9 Democratic gubernatorial primary between Lt. Gov. John Carney and state Treasurer Jack Markell.

Most of the early speculation is that the likeliest candidate for appointment to a possible vacant Biden seat is Beau Biden, his 39-year-old eldest son, who was elected Delaware attorney general in 2006. The younger Biden, a member of the Delaware Army National Guard, is about to enter active duty in Iraq.