Correction Appended

Nebraska Treasurer Don Stenberg on Tuesday morning will announce a run for Senate, setting up a Republican primary as the GOP attempts to unseat Sen. Ben Nelson (D). 

A Nebraska Republican operative confirmed Stenberg is jumping in, joining Attorney General Jon Bruning, who announced shortly after the November elections that he was exploring a run for Senate.

At least a couple of others have said they’re interested in the race. 

Nelson, in his second term, hasn’t made an official decision about running again. He had $1.4 million on hand at the end of 2010 and has begun hiring campaign staff. He is considered the most vulnerable Senator up for re-election in 2012, and Roll Call Politics rates this race a Tossup.

Bruning led a poll of potential Republican candidates by nearly 30 points in late January, and he has already begun drawing distinctions between himself and Nelson, especially on health care. On Monday he sent out a fundraising e-mail emphasizing fiscal differences.

“As if it weren’t enough that Ben Nelson received a $100 Million Kickback for his support of Obamacare, the non-partisan watchdog group, Citizens Against Government Waste has announced that Ben Nelson has ‘won’ the 2010 ‘Porker of the Year’ award,” he wrote. “Today is your chance to kick out the Porker of the Year, and send a message to Washington that you’ve had enough.”

Though a divided primary may help Nelson, the fact that the primary is in May means the Republican nominee would have time to recover before the general election. Former Nebraska Sen. Bob Kerrey, who chaired the Democratic Senatorial Campaign Committee in the late 1990s, told Roll Call the number of candidates getting in is a sign that Republicans agree Nelson is vulnerable. 

“It’s a state with a Republican majority, and I think the competitive primary’s indicative that they think they can win,” he said. 

Stenberg was easily elected state treasurer in 2010, and he previously served as the attorney general. He has lost three previous Senate runs. Most significantly, he narrowly lost to Nelson in the 2000 general election when the seat was open. 

Stenberg will make his announcement at 9:30 a.m. Central time at a former federal courthouse in Lincoln. The release notes that this is the room “where in 1974 Stenberg began his federal court law practice — a path that eventually led him to argue several cases in the United States Supreme Court.”

Correction: Feb. 28, 2011

The article misstated Stenberg’s political career. He did not serve as a state Senator. The article also mistakenly said he would announce in a federal courthouse. The location was once a courthouse but is not currently used in that capacity.