Lieutenant Gov. Brian Krolicki (R), who passed on a bid against Senate Majority Leader Harry Reid in 2010, is likely to strongly consider running, according to close associates, though he is close with Heller, and it is unlikely they would run against each other. Krolicki decided not to take on Reid after being charged with several felony counts for allegedly mismanaging state finances when he was state Treasurer; Krolicki was later exonerated, and newspaper editorial boards excoriated the Democratic attorney general who filed the charges.
2010 Senate nominee Sharron Angle has not ruled out a bid, though if Heller runs for Senate, his House seat would be an inviting target for Angle. Angle lost the primary to Heller in 2006.
On the Democratic side, Rep. Shelley Berkley would be the frontrunner if she chooses to get in; she has said she would decide by summer. If Berkley doesn't run, other potential Democratic candidates include Secretary of State Ross Miller, Treasurer Kate Marshall, Attorney General Catherine Cortez Masto and Controller Kim Wallin.
If Heller and Berkley wind up running against each other, the race will present Silver State voters with a stark choice between a rural conservative and a Las Vegas liberal. Nevada has voted increasingly Democratic of late, and President Obama won the state by 12 points in 2008. Hispanics have grown as a political force, giving Democrats a reason to hope. But Republicans have proven their ability to reach out to Hispanic voters in a state in which the governor, Brian Sandoval, is a Hispanic Republican.
On Monday, both sides said they were optimistic about the race.
"Nevada is now an open seat, and ripe for a Democratic pickup," said Guy Cecil, executive director of the Democratic Senatorial Campaign Committee. "It remains high on our target list. Whoever Republicans field as their candidate will have a tough time holding onto this seat in a blue-trending state with President Obama at the top of the ticket."
In his own statement, National Republican Senatorial Committee chair John Cornyn (R-Texas) said Republicans would be positioned to keep the seat. "Next year's Senate race in Nevada will now come down to a clear choice between two competing visions for our country -- between a Republican candidate who believes in smaller government, fiscal responsibility and creating good, private sector jobs, and a Democrat candidate who believes in keeping our country on the same reckless fiscal path of more government and higher taxes."
Ensign becomes the eighth senator to have announced he will not seek reelection next year: Ensign is the third Republican, following Sens. Jon Kyl of Arizona and Kay Bailey Hutchison of Texas.
This post was updated at 4:09 p.m.