LINCOLN, Nebraska (Reuters) - A state senator who entered Nebraska’s gubernatorial race as an independent said on Tuesday he had joined the Democratic Party to bolster his bid to unseat Republican Governor Pete Ricketts.

“I have 90 days before the May 15 Primary Election to introduce myself to Nebraskans registered in the Democratic Party and to earn their trust,” said Bob Krist of Omaha, who changed his voter registration on Monday.

Ricketts, whose family founded TD Ameritrade and owns the Chicago Cubs baseball team, won his first term in office in 2014, beating Democrat Chuck Hassebrook 57 percent to 39 percent.

Krist, a pilot and Air Force veteran, was appointed to Nebraska’s non-partisan single-chamber legislature in 2009 by then-Republican Governor Dave Heineman. Krist was elected to the seat as a Republican in 2010 and 2014 but announced in July he would run for governor as an independent.

However, a state law requiring the signatures of 10 percent of Nebraska’s 1.2 million registered voters and the lack of high-profile candidates among the Democrats led Krist to make the switch.

While Nebraska traditionally votes Republican, Ricketts could be at risk, said Randall Adkins, a political science professor at the University of Nebraska at Omaha.

“If anyone is going to unseat an incumbent, this is probably the year to do it,” Adkins said. “Krist is considered a credible candidate. He’s a veteran and a two-term incumbent with a conservative voting record in a red state. However, he is running against a well-financed incumbent governor.”

Nearly 49 percent of Nebraska registered voters are Republican, compared with about 31 percent Democrat and about 21 percent independent.

Nebraska has not elected a Democrat as governor since 1994, and a Democrat has not won a presidential nomination in the state since Lyndon Johnson in 1964. The highest-ranking Democratic office holder in the state is Lincoln Mayor Chris Beutler.

Rickett’s campaign spokesman Matthew Trail pointed to polling done by research firm Morning Consult from Oct. 1 through Dec. 31 that showed Ricketts with a 54 percent approval rating among voters.

“Throughout the odyssey of Senator Krist’s candidacy, Governor Ricketts’ campaign has remained focused on sharing the governor’s vision for a growing, stronger, more prosperous Nebraska,” Trail said.