“Republican leaders have unfortunately, but predictably, opted to take another senseless step to undermine the fair election process," Kerrey said in a statement. "It is an attempt to win by robbery what they would otherwise lose in an election. In the end, they will fail.”

Kerrey had moved from Nebraska to New York, where he led The New School, but returned to Nebraska last month before announcing his candidacy.

Kerrey listed his sister’s address as his home, but told the Omaha Herald that he “lucked out” the next day and found a place to live in the guest home of a friend, rendering the address on his voter registration obsolete.

Kerrey’s residency is not being challenged, Nebraska Secretary of State John Gale, a Republican, has said — the U.S. Constitution requires only that senators be a resident of their home state by the day they’re elected.

But Gale took umbrage with Kerrey’s quick change of circumstances, accusing him of being “cavalier” with Nebraska’s residency rules, and the state GOP is now looking to capitalize on this by saying that Kerrey broke his voter registration oath.

According to the Omaha Herald, Kerrey argued in a letter to Gale that his oath “was taken with the very serious intent of leaving private life, asking more of my family as all of us who enter public life do, and offering myself in service to the people of Nebraska and the United States of America. I do not need to be reminded of the seriousness of my oath since I have done so many times in the past.”

Even before Kerrey announced his candidacy, Republicans had already launched attacks against the former senator, dubbing him a “New York liberal” for his move to Manhattan. While it’s unlikely that the court challenge will keep Kerrey off the ballot in the fall, Republicans hope to gain by calling attention to his recent stint in New York.

Democrats had all but given up on holding on to retiring Sen. Ben Nelson's (D-Neb.) seat until Kerrey reversed his previous decision and announced in February he would enter the race, giving Democrats a viable and popular candidate in a conservative-leaning state.

—This story was updated at 2:57 p.m.