The conservative American Future Fund is running an ad against Nebraska Sen. Ben Nelson, one of the most vulnerable Democrats up for re-election in 2012.

The minute-long radio ad criticizes Nelson’s vote in favor of the health care reform law and urges him to vote for its repeal. The group is spending about $30,000 to run the ad for a week, according to founder Nick Ryan.

“Even after Nelson tried to cut back-room deals in his ‘Cornhusker Kickback,’ Nebraskans overwhelmingly opposed the liberal health care bill,” a female announcer says. “Tell Ben Nelson it’s not too late to put Nebraska first: Vote yes on repeal. Start over and get health care right.”

AFF ran a radio ad earlier this month in North Dakota not long before Democratic Sen. Kent Conrad decided to retire. Those ads ignited a short-lived air war, as a liberal group and Conrad himself each bought their own airtime. Conrad announced on Jan. 17 that he wouldn’t seek re-election, a move makes it more likely for a Republican to win his seat.

In Nebraska, Attorney General Jon Bruning (R) is already exploring a run against Nelson, and Treasurer Don Stenberg (R) may consider running, too. Bruning first explored a race to replace Sen. Chuck Hagel in 2008 but deferred to then-Gov. Mike Johanns, who ultimately won the seat. Gov. Dave Heineman (R) announced that he wouldn’t run against Nelson.

Roll Call Politics rates the Nebraska Senate race a Tossup.