Republicans are in a strong position to win the open Nebraska Senate seat thanks to Sen. Ben Nelson’s decision to retire, so Roll Call Politics is moving its rating from Tossup to Likely Republican.

Nelson was one of the most vulnerable Democrats in the country, and the Democratic Senatorial Campaign Committee already had invested more than $1 million in hopes of shoring him up before a difficult race.

Democrats say they have a shot, thanks in part to an ugly and crowded GOP primary.

Nebraska Gov. Dave Heineman said earlier this month he was being courted by Republicans in Washington and might give a primary bid a second look after initially ruling it out. Three others have been battling one another for the nod: Attorney General Jon Bruning, state Sen. Deb Fischer and state Treasurer Don Stenberg, who has been endorsed by conservative Sen. Jim DeMint (R-S.C.).

Democrats have hinted they will try to recruit former Sen. Bob Kerrey to run. Kerrey, traveling with family in India, did not directly answer a reporter’s question Tuesday about his intentions. The former Senator told Roll Call in an email that Nelson’s retirement “is a very big loss” for Nebraska.

The Omaha World-Herald reported that several Democrats might run, including state Sen. Steve Lathrop and former Lt. Gov. Kim Robak. But without a candidate officially on deck, the Democrats are in a position to lose this seat in November.

It’s a similar situation to what happened in North Dakota when Sen. Kent Conrad announced his retirement. Roll Call Politics also rates that race Likely Republican.

Republicans need a net gain of four seats next year to take outright control of the Senate. See the race ratings map here.