Washington ? The moderate middle is disappearing from Congress.

Evan Bayh is just the latest senator to forgo a re-election bid, joining a growing line of pragmatic, find-a-way politicians who are abandoning Washington. Still here: ever-more-polarized colleagues locked in gridlock — exactly what voters say they don’t like about politics in the nation’s capital.

Politics runs in cycles, and the Senate has seen flights of self-styled centrists before. In 1996, for example, 10 senators who could boast strong bipartisan credentials chose to retire rather than re-up. Many of them complained how lonely a place the middle ground of American politics had become. But to some, the center has become even lonelier.

More than their feelings are at stake. The moderates in the middle are the ones who tend to make deals and sometimes resolve standoffs blocking decisions that affect programs — not to mention taxes — that touch virtually every American.

Former Sen. William Cohen says what’s happening now is a continuation of the “hollowing out of the middle.” An article he wrote when he left his Senate seat in 1996, lamenting partisan gridlock, could just as easily be reprinted now, subbing his name for that of Bayh, the Indiana Democrat who announced on Monday he won’t run again.

“There is this sort of purging in both parties,” Cohen said in an interview. “They insist on moving to the left or moving to the right, and I think you’re seeing over the years the moderates have disappeared and continue to disappear.”

The few left in the middle can gain outsized power to decide the fate of closely fought issues. But that comes at a price more and more of them say is too high: crushing pressure to conform, shrill media barbs and the increased fight for cash to shape one’s own campaign narrative.

“I simply reached a conclusion that I could get more done to help my state and the American people by doing something in the private sector,” said Bayh, the two-term senator and former governor, on ABC’s Good Morning America on Tuesday. “Real accomplishments in a real way.”

That’s an extraordinary statement on the anniversary of the $787 billion stimulus package that was supposed to energize the economy. Rather than heed President Barack Obama’s appeal for pragmatism, Congress is losing its value as a problem-solver and becoming more unworkable, Bayh said.

Polls say voters hate that about national politics. Lawmakers profess to dislike the polarization, too, but they still engage in it, on the House or Senate floor, in private meetings, or both. And on the campaign trail, the truth is there’s cash to be made by taking sides and, in effect, becoming a dependable brand.

“If you’re on either fringe of the party, you have an easier time raising money,” said one who would know, Sen. Arlen Specter, who left the GOP for the Democrats when he found he could not win a Pennsylvania Republican primary. “I have to work a lot harder than somebody who has an ideological base.”

And moderates? An endangered species?

Moderates, said Darrell West, vice president and director of governance studies at the Brookings Institution, are “going the way of the dinosaur.”

“Soon we’re going be able to go to museums to the see the skeletons of the centrists and learn about what they were,” West said.