Kansas Democratic candidate Barry Grissom ended his campaign Thursday for the U.S. Senate seat held by retiring Sen. Pat Roberts, departing one day after state Sen. Barbara Bollier announced her bid for the party's nomination.

Grissom, a former U.S. attorney for Kansas, sought to transform law-and-order credentials into a competitive bid, but struggled to gain traction in a heavily red state. Kansans favored President Donald Trump by nearly 20 points in 2016, but the state's voters elected Democrat Sharice Davids to a seat in the U.S. House and Democrat Laura Kelly as governor in November 2018.

"While I’m incredibly proud and humbled by the support friends have given me during this campaign, I’m announcing that I no longer intend to seek the Democratic nomination to the U.S. Senate," Grissom said in a statement.

Bollier represents Mission Hills in voter-rich Johnson County, the foundation of Davids' campaign for Congress and a significant factor in Kelly's victory. Grissom endorsed Bollier's candidacy for the U.S. Senate.

"Working to win our U.S. Senate seat for the first time in more than 80 years has always been a mission that’s bigger than just me and I know a drawn-out primary would be harmful to our fight," Grissom said. "I’ve come to believe the most important thing I can do to help us win this race is to step aside, and ask for all those that want to challenge Kansas’ status quo to unite with me in supporting Barbara Bollier."

She said Grissom's endorsement was welcomed and promised to be a voice of reason in Washington, D.C.

"This is a critical year for our state and our country," Bollier said. "As a doctor, my lifelong mission has been to help people. I will continue to stand up for our families to find common ground and reasonable solutions to the challenges impacting us each day, like health care and child care."

Bollier decided two years ago to change her affiliation from Republican to Democrat. She said the GOP's views on education, health care and other issues no longer fit with her approach to government.

Republican candidates in the U.S. Senate primary race include U.S. Rep. Roger Marshall, of the 1st District; former Secretary of State Kris Kobach; former NFL player Dave Lindstrom; Senate President Susan Wagle and commentator Bryan Pruitt. Bollier will compete with Usha Reddi, of Manhattan, for the Democratic nomination.

Grissom served as U.S. attorney for Kansas from 2010 to 2016. His tenure in that office has been clouded by an emerging scandal in which federal prosecutors gained access to recordings of attorney-client conversations at the Leavenworth Detention Center. It's a facility operated by CoreCivic, formerly known as Corrections Corporation of America.

One review of the evidence indicated the U.S. attorney's office accessed more than 1,400 attorney telephone calls between 2010 and 2017. The assessment indicated a prosecutor had a 28% chance of gaining access to attorney-client conversations.

"I’m outraged by these prosecutors, and had I known, they would have been immediately terminated," Grissom said in August. "But these prosecutors knew they were up to no good, which is why they purposefully violated the law, their ethical obligations, and hid their actions."

Grissom had been endorsed by SMART Kansas Union, former Wichita Mayor Carl Brewer, former Kansas City, Kan., Mayor Mark Holland and three former Kansas Democratic Party chairmen.