Rep. Dan Boren (D-Okla.) is one of 25 House incumbents retiring this year. And unlike most of his fellow retirees, his exit puts a previously safe seat in play. (Linda Davidson/Washington Post)

New York Democratic Rep. Edolphus Townsretirement announcement today makes him the 25th House retirement of this cycle. Add in the 10 Senate retirements, and you’ve got the most combined retirements since 1996, when Democratic lawmakers retired in droves after the Republican Revolution of 1994 (and many Republicans joined them).

Add in another 15 House members running for other/higher office, and the next Congress is already promising to include plenty of turnover — with 50 out of 535 members (nearly 10 percent) already leaving their seats.

Filing deadlines have passed in 30 of the 50 states so far, meaning there may not be that many more retirements to come. Incumbents are generally urged by their parties to retire early in the cycle so that capable successors can step forward, though some — like Towns — will wait until right before the filing deadline.

But the picture is almost complete. In 2010, for example, only two House incumbents announced their retirements after this point in the cycle.

The most retirements in one cycle over the last four decades was in 1992, when 52 House members and seven senators called it quits, according to Roll Call’s “casualty list.” (For more on historical retirement numbers, check out Roll Call’s great “Casualty list.”)

The smaller surge in retirements this cycle shouldn’t be much of a surprise; retirements tend to happen more in election years following a big shift in the Congress (like 1996) and more in elections following redistricting (like 1992). The 2012 election has both of those things going for it.

Indeed, the confluence of those two circumstances makes it a little surprising that there haven’t actually been more retirements — particularly in the House. But, of course, there’s still some time on the clock.

Overall, the retirements have hurt Democrats more, with 15 of the 25 House retirements and six of the 10 Senate retirements coming at their expense.

While the retirements have stacked up in both chambers, though, the actual impact on the election will be much more pronounced in the Senate.

While only a handful of the 25 House retirements come from districts that will be competitive this year, most of the 10 retirements in the Senate come in swing (or semi-swing) states.

In the House, fewer than 10 of the 25 retirements are occurring in districts that could change hands. What’s more, because of redistricting, about half of those members were likely to lose anyway.

In the end, the only retirements that are likely to matter are those of Reps. Mike Ross (D-Ark.), Dan Boren (D-Okla.), Jerry Costello (D-Ill.) and Tim Johnson (R-Ill.). In those seats, the other party has significantly better odds at winning the seat now that they don’t face the incumbent. (That’s not to say the seats will flip party control but that the chances are increased with no incumbent running.)

The other retirees — members like Reps. Brad Miller (D-N.C.), Heath Shuler (D-N.C.), Jerry Lewis (R-Calif.), David Dreier (R-Calif.) and Elton Gallegly (R-Calif.) — all faced tough odds after redistricting and mostly succumbed to their new political realities when they called it quits.

The retirement picture has had much more of an impact on the Senate side, where the retirements of Democratic Sens. Daniel Akaka (Hawaii), Jeff Bingaman (N.M.), Kent Conrad (N.D.), Herb Kohl (Wis.) and Ben Nelson (Neb.) expanded the playing field significantly in the GOP’s favor in 2011 and early 2012.

Then recently, when Sen. Olympia Snowe (R-Maine) retired, the GOP’s task in regaining the Senate majority became more difficult.

The 10 Senate retirements is also the most since 1996, when 13 senators called it quits.

The Senate playing field now seems set but there may be more House retirements in the weeks and months ahead. If those retirements come from districts like Towns’,which went 91 percent for President Obama in 2008, the practical effect on the balance of power will be minimal.

The key to watch will which party — if either — is afflicted by surprise retirement in swing seats.