When the clock strikes midnight on the 111th Congress, the Senate will complete a two-year span in which hundreds of years of institutional knowledge, experience and star power will have exited the chamber to make way for a crop of new lawmakers.

The generational exodus began in the days and weeks after Election Day 2008, when one Senator resigned to become president, another vice president and a third ­— Sen. Ted Stevens (R-Alaska) — officially lost his bid for a seventh term when mid-November vote counting confirmed his defeat.

It continued when the 111th convened as two Senators became Cabinet secretaries. Then came the deaths of Sen. Edward Kennedy (D-Mass.), taking with him the last vestige of a consequential familial Senate dynasty, and Sen. Robert Byrd (D-W.Va.), who was in the middle of his ninth term.

For institutionalists, it got worse as the campaign season progressed. Sen. Chris Dodd (D-Conn.) announced his retirement, opting not to face a hostile electorate. Sen. Bob Bennett (R-Utah), like Dodd, the son of a former Senator, chose to fight for his seat, but he failed to earn a spot on his state’s primary ballot. Longtime Sen. Arlen Specter (Pa.) left the GOP to improve his electoral prospects, only to lose a Democratic primary, while Sens. Russ Feingold (D-Wis.) and Blanche Lincoln (D-Ark.) lost in the Nov. 2 general election.

Sen. Byron Dorgan (N.D.), a third-term Democrat who chose retirement over running for re-election, said the heavy turnover has positive implications as well as a few drawbacks. The incoming Members bring with them fresh energy and perspectives on pressing issues, although the loss in policy and political expertise garnered over several years can be detrimental to the Senate’s ability to function as the Constitution intended, Dorgan said.

“I do think it is in many ways good for the Senate to have new people coming in. It’s also important for them to understand the traditions and the culture and why it is the body that it is,” Dorgan said last week. “Those who have been around here a long time have a vast reservoir of knowledge on issues … and that institutional knowledge is important for a body like the Senate.”

First elected in 1992, Dorgan serves as Indian Affairs chairman and holds the gavel on two subcommittees. He also heads the Democratic Policy Committee, an appointed position within the Democratic Conference leadership.

In all, eight Senators who have served a combined 23 terms are voluntarily retiring this year. Among them are four former governors, including two-term Sen. Evan Bayh (D-Ind.), four-term Sen. Kit Bond (R-Mo.), three-term Sen. Judd Gregg (R-N.H.) and two-term Sen. George Voinovich (R-Ohio). An additional four men who were appointed did not stand for re-election. Another four Members with a combined 13 terms of service are leaving the chamber after losing their bid for re-election.

Until late 2008, President Barack Obama, Vice President Joseph Biden, Secretary of State Hillary Rodham Clinton and Interior Secretary Ken Salazar all served in the Senate. Kennedy, who passed away in the summer of 2009, and Byrd, who died in June of this year, are both considered among the most effective legislators to ever serve in the chamber.

Byrd was the architect of key Senate rules governing the filibuster and the chamber’s regulations for approving a federal budget.

On a lighter note, the retirement of two-term Sen. Jim Bunning (R-Ky.) will see the exit of a Hall of Fame baseball player who is one of the few to have pitched a perfect game.

“Some of it is generational,” Senate Historian Don Ritchie said, explaining the turnover. “Older Members are retiring or dying. Some of it is that political winds are shifting.”

“We have had periods where a large percentage of the Senate was in its first term,” Ritchie added. “Then those people tend to get re-elected and stay awhile, and the cycle starts all over again.”

Come January, 16 freshmen are set to be sworn in, bringing the total turnover in the Senate to 40 since the Democratic wave election of 2006 — 10 were elected that year and 14 in 2008. And yet, previous elections have seen even greater upheaval.

The largest freshman class since World War I was in 1946, which, like this year, was a big Republican year. That year, the GOP captured 55 House seats and 12 Senate seats to retake control of Congress for the first time since 1928. Total size of the freshman Senate class coming out of that election: 22, although two of those elected had previously served in the Senate.

This year’s freshman class includes Dan Coats (R-Ind.), who retired from the Senate in 1998.

More recently, in the elections of 1976, 1978 and 1980, the total number of new Senators to emerge from those contests numbered 56.

In 1976, the first presidential election after the Watergate scandal, 10 new Democrats and eight new Republicans were elected. In 1978, President Jimmy Carter’s midterm year, nine new Democrats and 11 new Republicans were elected. And, in the 1980 election that saw Ronald Reagan become president, the GOP regained control of the Senate with a freshman class of 16. Only two new Democrats were elected that year.

“This institution survives,” Dorgan said. “You have the difficult times and easier times. But the institution survives those of us who come and go.”