It’s officially a trend—over the past week, three U.S. senators have announced that they will forgo running for re-election in 2012. Texas Republican Sen. Kay Bailey Hutchison was first out of the gate with her announcement last Thursday that she would not seek a fourth term. Then, on Tuesday, North Dakota Democratic Sen. Kent Conrad announced that he planned to retire and was followed less than twenty-four hours later by Connecticut Sen. Joe Lieberman, a Democratic-leaning Independent.

In addition to Conrad and Lieberman, there are twenty-one other members of the Democratic Senate caucus up for re-election in 2012 compared with just ten Republicans, a consequence of the 2006 wave election in which Democrats captured seats in generally Republican-leaning such as Montana as well as swing states like Ohio. 

Potential candidates in all three states are jockeying for position, and party eminences have begun to make their preferences known. In Texas, former President George H. W. Bush has announced his support for Secretary of State Roger Williams as a successor to Hutchison. Williams will face serious opposition from Lt. Gov. David Dewhurst, who announced his intention to run last Friday. Texas Railroad Commissioner Michael Williams is likely to announce his own Senate bid next week. Another potential contender: U.S. Rep. and former presidential candidate Ron Paul, who came in a close second to Dewhurst in a recent poll of the state’s Republicans. 

On the Democratic side, former Houston Mayor Bill White expressed interest in the Senate before mounting his unsuccessful 2010 bid for governor. A February 2009 poll found White trailing Dewhurst by 5 points. However, Nate Silver speculated last Thursday that it any Democratic candidate for the seat will face significant obstacles. 

In North Dakota, where Conrad had been running radio advertisements defending his record as recently as last week, Republican state Public Service Commissioner Brian Kalk had expressed interest in challenging the Democratic incumbent before he announced his retirement. Among the other Republicans considering a run are state Attorney General Wayne Stenehjem, Tax Commissioner Cory Fong, Gov. Jack Dalrymple and Lt. Gov. Drew Wrigley.

Conrad’s retirement opens the field to potential Democratic candidates, though they face steep odds as well. In the 2010 race to replace retiring North Dakota Democratic Sen. Byron Dorgan, Republican John Hoeven defeated Democrat Tracy Potter by a whopping 54 points.

Lieberman’s decision was the most closely watched of the three. The former Democratic vice presidential nominee angered many in his party with his outspoken support for the Iraq War and lost the Democratic Senate primary in 2006 to Greenwich Town Selectman Ned Lamont. Lieberman came back to win in the general election as an Independent, buoyed by strong support from the state’s Republicans. This time around, he was likely to face a stronger Republican nominee in addition to continued strong opposition from Democrats, making his path to re-election increasingly uncertain. Indeed, an internal poll released Tuesday by former Connecticut Secretary of State Susan Bysiewicz, who announced her candidacy the same day, showed her leading in several Democratic primary scenarios as well as a three-way general election against Lieberman and either Linda McMahon, the 2010 Republican Senate nominee, or Tom Foley, the 2010 Republican gubernatorial nominee.

Of course, Bysiewicz is likely to have competition in the Democratic primary before she even gets to the general election. Rep. Chris Murphy (CT-5) has been signaling his intention to run for several weeks and, after Bysiewicz released her internal poll, he released a statement indicating that he would announce his intentions soon.

In recent weeks, speculation has increased that several embattled Democrats—including Nebraska Sen. Ben Nelson and Virginia Sen. Jim Webb—will bow out of potentially bruising re-election fights in 2012. On the Republican side, the threat to incumbents comes primarily from far-right primary challenges, as in the case of Maine Sen. Olympia Snowe and Utah Sen. Orrin Hatch. Nevada Sen. John Ensign, who has been embroiled in an adultery scandal, would presumably be vulnerable as well should he choose to run again.

Noah Rothman is the online editor at C&E. Email him at nrothman@campaignsandelections.com