U.S. Sen. Olympia Snowe, R-Maine, shocked her constituents and Republican colleagues on Tuesday when she announced her resignation, ending an almost 20-year career in Congress' upper chamber.  

As one of nation's most recognizable moderate Republicans, Snowe's decision to suspend her career -- which she credited to extreme political polarization that led her to reconsider how productive an additional term would be -- made national headlines. However, Snowe, along with U.S. Rep. David Dreier, R-Calif., who announced his own retirement on Wednesday,  are only the most recent member of Congress to announce they are throwing in the towel ahead of November's election.

Senate's Democratic Majority Could Be Threatened

As of now, 10 senators have announced they plan to retire their seat at the end of the 112th Congress -- six Democrats, three Republicans and one independent, Sen. Joseph Lieberman of Connecticut.

Although none of the other senators have directly cited Washington, D.C.'s gridlock-causing partisanship as a reason for their retirement, some have at least hinted at it. For instance, in a statement announcing his decision to leave the Senate, Ben Nelson, D-Neb., wrote that Simply put: It is time to move on.

Others Democrats, such as Daniel Akaka of Hawaii and Jeff Bingaman of New Mexico, have also indicated that a tough reelection influenced their decision, although both of the senators also said the significant time they had already spent in the chamber -- 22 years and almost 30 years a piece -- led them to believe it was time to sit aside.

The seats being vacated by Nelson, as well as Democrats Sens. Kent Conrad (N.D.), Jim Webb (Va.) and Herb Kohl (Wisc.) could be vulnerable to Republican in those GOP-leaning states. Still, Snowe's seat in Maine -- typically a left-leaning state -- could go over to Democrats, as well as Lieberman's open seat in Connecticut.

Republican Sens. John Kyl of Arizona and Kay Bailey Hutchinson of Texas are also retiring at the end of the 112th Congress.

In House, Several Leaving to Run For Senate

In the U.S. House, 12 Democrats --including party heavyweight Barney Frank of Massachusetts -- and nine Republicans will retire, in addition to the 11 congressmen who are leaving the lower chamber to run for the Senate, and another four who are running for another office. One of those includes Ron Paul, R-Tex., who announced he would not seek reelection after 24 years in the House as he launched his third bid for the U.S. presidency.

While that only makes a small portion of the House, which boasts 435 members, considerably more seats are up for grabs. In 2010, when the Tea Party revolution boosted Republicans to the chamber's majority leader, 56 representatives lost their re-election bid. 54 of those representatives were Democrats.

Two years down the road, many of the Tea Party Republicans who were victorious in 2010 could face serious challenges from disgruntled constituents. Congress has an all-time low approval rating for a reason: the American public doesn't think their elected representatives are doing their jobs well.

The House members who plan to vacate their seats for the Senate include:

Reps. Todd Akin, R-Mo., Tammy Baldwin, D. Wisc., Rick Berg, R-N.D., Shelley Berkeley, D-Nev., Joe Donnelly, D-Ind., Jeff Flake, R-Ariz., Martin Heinrich, D-N.M., Mazie Hirono, D-Hawaii, Connie Mack IV, R-Fla., Christopher Murray, D-Conn., Denny Rehberg, R-Mont.

Click here to see a full list of Roll Call's Casualty List of the 112th Congress.