American Crossroads, a Republican-affiliated outside group, is once again spending money on Nebraska's Senate race. But this time its target is former Sen. Bob Kerrey (D-Neb.), who has returned to the state and is mulling a bid to return to Congress.

The group is running a small, $30,000 radio ad buy hitting Kerrey for living in New York City for the last decade and voting for "the largest tax increase in history."

Crossroads spent hundreds of thousands of dollars against Sen. Ben Nelson (D-Neb.) last year in an effort to convince him not to run for reelection. Now it is doing the same with Kerrey, who currently lives in New York but is back in Nebraska discussing a possible candidacy with state Democratic leaders.

Nelson's retirement was a bad development for Democrats, who are trying to hold on to a narrow 53-47 margin in the Senate. They have 23 seats to defend this election to Republicans' 10, and retirements in Republican-leaning Nebraska and North Dakota have made that an even tougher challenge. Kerrey is likely the only Democrat who could make this a tough race for Republicans, and they're trying to persuade him not to run.

Listen to the ad here: