This story has been updated.

As Tuesday's Mississippi Republican senate primary drew closer, coverage of the race increasingly addressed the age of 76-year-old incumbent Thad Cochran, who has been in the Senate since December 1978. Before that, he served in the House, from 1972 until his promotion. One senator was born the year that Thad Cochran won federal election for the first time.

U.S. Sen. Thad Cochran, R-Miss., reads over the ballot in a privacy booth prior to voting absentee in Oxford, Miss., Saturday, May 31, 2014.  (AP Photo/Rogelio V. Solis)

In other words, Thad Cochran has been in Washington, D.C., a long time, so long that only 11 people remain on the Hill who have been there longer or just as long as he has. That's 12 legislators who were serving before 1979, out of 535.

Five of those politicians are leaving Capitol Hill this year, many citing frustrations with the whippersnappers who now outnumber them as the reason they'd rather not stay.

One senator who can match Cochran for longevity decided to retire this year. Sen. Carl Levin (D-Mich.) announced he was leaving the Senate in 2013. The Detroit Free Press's associate editor wrote of his retirement,

He clearly loved the Senate as it was, arcane rules and all, rather than the partisan snake pit it was becoming. The avuncular Levin counted Republican and Democratic colleagues among his close friends -- political differences not withstanding -- and clearly was unhappy with the chamber's recent atmosphere.

That may have had as much as anything to do with his decision to call it quits. He is a senator from another time, when the Senate really was something of a very exclusive club but also earned its description as "the world's greatest deliberative body."

A bit romantic, but it seems like many other soon-to-be retirees from the lower chamber feel the same.

When Rep. John Dingell (D-Mich.) announced his intention to retire in February. Part of the reason is that he now finds "serving in the House to be obnoxious."

Rep. Henry Waxman (D-Calif.) has been in the House since 1975. He announced his retirement this January by noting, “There are elements of Congress today that I do not like." He added that he was "not leaving out of frustration with Congress," though. "After 40 years in Congress, it’s time for someone else to have the chance to make his or her mark, ideally someone who is young enough to make the long-term commitment that’s required for real legislative success."

Rep. George Miller, also a California Democrat, said the state of Congress had no influence on the timing of his departure. His former aide told the Washington Post in January Miller was at least annoyed by the changes he'd witnessed. “He talked about how the climate is now, compared to 10 years ago. Particularly with the tea partyers; he said they don’t even make eye contact with you. It just makes it less fun.”

Republican Tom Petri, who has represented Wisconsin in the House since 1979, is retiring at the end of the year. He has only faced two primary challengers over the course of his career, but a tea party conservative had announced his intention to run against him in 2014. "Sooner or later, you either are booted out, die or retire," Petri said in April after he announced his time was up.

Booted out, dead, or retired. Harsh, perhaps, but you can't say it isn't true. Many of the eight remaining legislators who hope to keep their jobs past December have faced significant hurdles or are making exit plans.

Detroit Rep. John Conyers, who has been in the House since 1965, almost didn't make it on to the ballot this year after his campaign failed to get enough signatures on his re-election petition. Rep. Nick Rahall (D-W.Va.), who's currently running his 20th campaign, holds one of the congressional seats Republicans are most eager to pick up. Sen. Orrin Hatch (R-Utah) has said he doesn't plan on running for re-election in 2018. The tea party challenger who he faced off with in 2012 might have something to do with it.

And then there's Thad Cochran, who in 1990 "was in such a strong political position this year that no serious candidate chose to oppose him." In 2014, he faced a serious primary challenge from the right, like many of his fellow old timers. The primary was especially nasty, and is so close that it looks like Cochran and his tea-party opponent are headed for a runoff.

That leaves Sen. Pat Leahy (D-Vt.), Rep. Don Young (R-Alaska) and Rep. Jim Sensenbrenner (R-Wis.) as the only members of the Cochran and older set who aren't immediately imperiled.

For those three legislators, the experience they've accumulated over the decades is the reason they keep winning races. For most of the other members of the elder dozen, the experience they've accumulated over the decades is why it has become so impossible to stay. Accusations of going stale are the No. 1 cause of career death for those who have been in Congress longer than we can remember. Becoming a synecdoche for the problems of Congress writ large are the second leading cause. When Americans cite dissatisfaction with government as one of the most important problems facing the country, it's not surprising that the legislators who have been part of the scenery so long they've start to blend in with the problem are becoming an endangered species.

In May, Club for Growth president Chris Chocola released a statement that asked, "What's Thad Cochran scared of? ... Is he afraid that Mississippi voters might realize he's been there too long?" Chocola said something similar about Dick Lugar two years ago, when the tea party challenger-encouraging group successfully took down the senator, who had served since 1977. "There is no reason to believe that six more years from Richard Lugar in Washington will be different from the past 35 years," Chocola wrote on the National Review Web site. "It’s time for him to come back home to Indiana."

When Independent Bill Bloomfield ran against Waxman in 2012, he said, “Yeah, he’s been there for 38 years, but I have an answer: ‘How’s that working out?’”

Given the country's current mood, for these 12 legislators it works out worse and worse as the years pile up.

 Correction: An earlier version of this article stated that retiring Rep. Tom Petri was a Democrat. He is a Republican.