Thirteen candidates — four Republicans, eight Democrats, a Libertarian and a Constitution aspirant — are seeking party nominations for lieutenant governor in what has become a high-profile race in the Aug. 7 primary.

The GOP incumbent, Peter Kinder, is facing his strongest primary challenge since being elected in 2004, from state Sen. Brad Lager, of Savannah in northwestern Missouri.

Lager is getting help from a newly formed political action committee that has paid for recent ads in the Joplin market accusing Kinder of missing a tie vote in the Missouri Senate because he was at the Horny Toad bar at Camden on the Lake Resort.

The ad claims: “The last time there was a tie vote, Kinder was spotted at a bar: the Horny Toad bar at the Lake. Kinder’s aggressive behavior with a stripper forced her to end contact. Questionable behavior? It’s last call for Peter Kinder. Time to cut him off.”

The Kinder campaign has denied the charge that Kinder was at the bar. Kinder’s campaign said he was at the hotel to speak to a group of seniors in an adjacent ballroom.

In response, Kinder has fired back with an ad criticizing Lager’s vote to accept federal stimulus money from the 2009 American Recovery and Reinvestment Act. Kinder also is touting his opposition to “Obamacare.’’

When Lager announced late last year that he would make a bid for the post, Kinder was readying his own campaign for governor. After a subsequent spate of press coverage of previous activities, Kinder decided against a run for governor but to seek re-election to a third term as lieutenant governor.

Among the disclosures against Kinder: that he had billed the state for numerous stays at St. Louis-area hotels and was a frequent customer at a strip club in the mid-1990s while serving in the state Senate. He later reimbursed the state more than $50,000 for the hotel stays.

Lager, 37, a former state representative and former member of the Maryville City Council, was a 2008 nominee for state treasurer. After running unopposed in the Republican primary, he lost the November general election to Clint Zweifel, the Democratic nominee. He is term-limited by Missouri law from running for his state Senate seat again.

Lager has stockpiled nearly as much money as the incumbent he is trying to topple. According to the Missouri Ethics Commission, he has received two $250,000 donations in his bid for lieutenant governor. The donations, made late last year, were from Stanley Herzog, via Herzog Contracting, St. Joseph, and from David Humphreys, via TAMKO Building Products, Joplin.

Kinder, 58, a Cape Girardeau attorney, was the only Republican in Missouri to win statewide office in 2008.

Among the eight Democrats seeking their party’s nomination for the seat are three candidates who appear to be the front-runners. They are Susan Montee, Judy Baker and Sara Lampe.

All of the major candidates are running on campaigns that focus on seniors, veterans, education and jobs.

Montee, 53, a St. Joseph attorney and accountant, is the former chairwoman of the State Democratic Party and a former state auditor. She won the state auditor’s race in November 2006. She was defeated in the 2010 general election by Tom Schweich, a Republican.

Montee, in a statement from her website, said she would reform two agencies — the Missouri Housing Development Commission and the Missouri Development Finance Board, based on what she learned after doing audits of those agencies. She said the reforms could contribute to job growth.

“As state auditor, I investigated both of these influential entities and discovered financial inefficiencies and problems with the way their boards operate,’’ she said.

As lieutenant governor, she said she would have the opportunity to implement her recommended changes.

Lampe, 63, an educator and state representative from Springfield, was born and raised in Cassville. Elected to the Missouri House of Representatives in 2004, she is term-limited by Missouri law from running for a House seat in 2012.

Lampe, on her website, said she would be accessible, available and accountable to all Missourians, and “champion best practices for seniors, veterans and in education, and ensure the office reflects the diversity of the state.’’

She said there is a connection between education and economic prosperity. “An educated, talented work force is key to economic progress,” she said. “Missouri deserves that work force, and our children deserve that quality education.’’

Baker, 52, of Columbia, is a former state representative. She was first elected to the Missouri House in November 2004. In 2008, she was a nominee for the U.S. House of Representatives from Missouri’s 9th Congressional District. Following her defeat in that race, Baker returned to the private sector, teaching at Columbia College until November 2009, when she was appointed Region 7 director for the U.S. Department of Health and Human Services. Baker would hold that position until resigning in early December 2011 to run for lieutenant governor.

During the Joplin tornado in 2011, the U.S. Department of Health and Human Service’s response was under Baker’s direction. She helped coordinate a child care task force and a mental health care task force, and provided oversight of Medicare billing to ensure funding continued to flow to hospitals.

Baker, on her website, said, “I’m running for lieutenant governor so that I can be part of the solution to some of Missouri’s most pressing problems — from finding ways to create jobs and grow our economy, to helping Missouri’s returning veterans lead rich productive lives; to providing our seniors the support they need to live vibrant and independent lives for as long as possible, and to fighting for priorities like Missouri’s schools.’’

Also appearing on the Republican ballot will be Mike Carter, a former Wentzville municipal judge, attorney, Realtor and 2008 candidate for the post; and Charles Kullmann, of St. Louis.

Also appearing on the Democratic ballot will be Bill Haas, an attorney and a member of the St. Louis School Board; Fred Kratky, of St. Louis, a former state representative; Jackie Townes McGee, of Hayti, a former state representative and attorney; Becky Plattner, presiding commissioner of Saline County, and a former city councilor and clerk for Grand Pass; and Dennis Weisenburger, of St. Joseph.

The Constitution candidate is Cynthia Davis, a former Republican and former state representative from O’Fallon. The Libertarian candidate is Matthew Copple, a software developer from Gladstone.