Washington ? The roots of political gridlock in Washington and of the hyper-partisanship dividing “Red and Blue America” came into view on successive days at the end of April.

On April 27, Sen. Arlen Specter of Pennsylvania, a veteran moderate Republican, survived a primary challenge from conservative Rep. Pat Toomey by only 12,600 votes out of more than 1 million. A weakened Specter still faces a formidable challenge in November from Democratic Rep. Joe Hoeffel, with the possibility of another right-winger, National Constitution Party chairman James Clymer, running as an independent.

One day earlier, six-term Rep. Jack Quinn of Hamburg, N.Y., a Republican who has held a strongly Democratic district by dint of his personal support from organized labor, unexpectedly announced his retirement at the end of this year, creating an opportunity for a Democratic takeover.

Like the retirement plans announced earlier in this cycle by five southern Democratic senators, whose seats are prime targets now for the GOP, the developments in Pennsylvania and New York illustrate how the ideological lines between the parties are being etched ever deeper.

A Congress which for decades in the middle of the 20th century saw its agenda set by a centrist coalition of conservative Democrats and moderate Republicans has seen that center steadily depopulated by tugs from opposite political poles.

Scores of House and Senate seats once held by moderate-conservative southern Democrats have moved to the Republicans. And voters in New York, New England and the northern tier from Michigan west to Washington, who once sent progressive Republicans to the House and Senate, now send Democrats instead.

The dynamics of the process are well recounted in the latest book by Nelson Polsby, a University of California-Berkeley political scientist, “How Congress Evolves: Social Bases of Institutional Change.”

This realignment process has been so gradual that its effects are often overlooked. But when we awoke to the fact that the leaders of the newly installed Republican majorities in 1995 came from Georgia and Mississippi, while the last two Democrats to serve as Senate leaders came from once-Republican Maine and South Dakota, the dramatic turnaround was unmistakable.

Still, even voters attentive to politics often complain about the “excessive partisanship” in Congress without understanding why it has developed. The reality is that the party caucuses have become much more internally cohesive — Republicans, conservative; Demo-crats, liberal — while the policy differences between the parties have become much clearer and better-defined.

We know with some certainty that process will continue this year, no matter which party does better in November. The Republicans running for the open Democratic Senate seats in North Carolina, South Carolina, Florida, Georgia and Louisiana are — almost without exception — more conservative than the men they are seeking to replace, even though two of the Democrats, Sens. Zell Miller of Georgia and John Breaux of Louisiana, vote more often with the Republicans than the vast majority of their colleagues.

The GOP is likely to win some of those seats and is competitive in all of them. The departure of those southern Democrats will tilt the Democratic caucus further to the left, a process already far advanced in the House, where Democrats are led by Rep. Nancy Pelosi, a classic San Francisco liberal. And the trend will be furthered in the House by the Republican redistricting of Texas, aimed at eliminating half a dozen moderate to conservative Democrats in that delegation.

The reverse is happening among House Republicans. Quinn is rated by National Journal as one of most liberal Republicans in the House. The retirements of Quinn and four others, Reps. Doug Bereuter of Nebraska, Doug Ose of California, Porter Goss of Florida and Amo Houghton of New York, will further diminish the already small band of moderates in the House Republican Conference. The Republican nomination to succeed Ose has been captured by Dan Lungren, a staunch conservative who once ran for governor of California, and other conservatives are challenging in Nebraska and New York.

Specter is the most senior among the handful of Senate Republican moderates, and therefore he became the prime target for conservative activists in groups such as the Club for Growth, whose president, Stephen Moore, called it “a race for the heart and soul of the party.”

Despite his huge financial advantage and open support from President Bush and the national party establishment — who feared Toomey would lose the seat if nominated and be a drag on the president’s bid for Pennsylvania’s electoral votes — Specter was barely able to survive.

He is a tenacious campaigner and not to be underestimated in November. But the trend lines of history are working against him. As the polarization continues in both parties, moderate Republicans and conservative Democrats have become endangered species.

David Broder is a columnist for Washington Post Writers Group.