Former President Donald Trump just offered up his endorsement in a key congressional race that could not only help ensure Republican control over the House in the next term but also help ensure those Republicans are supportive of his agenda if he wins a second term in the White House.

Trump announced in a Truth Social post on Monday that he had picked Dr. Bob Onder, a former state senator, to win Missouri's 3rd Congressional District, according to local media outlet KMOV.

The former president praised Onder as an "incredible America First Patriot" ahead of Missouri's August 6 congressional primary and bashed one of the candidate's chief Republican rivals, former State Sen. Kurt Schaefer, as being "WEAK ON MAGA."

Trump's endorsement of Onder

In his Truth Social post on Monday, former President Trump wrote, "Dr. Bob Onder is an incredible America First Patriot running for Congress in Missouri’s 3rd Congressional District. A highly respected physician and attorney, Bob was a delegate for us in 2016, and has been an incredible voice for MAGA and the Great People of Missouri!"

"As your next Congressman, Bob will fight tirelessly to Stop Inflation, Grow the Economy, Secure the Border, Support our Military/Vets, Protect our always under siege Second Amendment, and Restore American Energy DOMINANCE," he continued.

"His opponent, Kurt Schaefer, is WEAK ON MAGA. That’s all you have to know!" Trump added. "Dr. Bob Onder has my Complete and Total Endorsement -- He will not let you down!"

The endorsed and the spurned respond

The Missouri Independent reported that former President Trump's endorsement of Dr. Onder for Missouri's 3rd Congressional District only singled out Schaefer for criticism and not any of the other candidates in the seven-way primary battle to replace outgoing incumbent Rep. Blaine Luetkemeyer (R-MO), who surprisingly announced he would not seek another term in January.

In response to the endorsement, Onder said he was "honored" to receive it and stated, "In 2025, we will reverse the disastrous policies of Joe Biden and the Democrats, and restore America First principles. President Trump was the greatest President of my lifetime and I look forward to being his ally in Congress to pass his America First Agenda and Make America Great Again."

Schaefer, who is close with and endorsed by Luetkemeyer, fired back by highlighting the fact that he has supported the former president from the beginning while Onder had initially backed Sen. Ted Cruz (R-TX) during the 2016 presidential primary.

"Unlike Bob Onder, I’ve always supported President Trump," Schaefer said. "I’ll use my experience as a proven prosecutor with a record of enforcing the law to fight for President Trump and his agenda. Together, we’ll restore law and order, expose D.C. corruption, and secure the border."

Onder and Luetkemeyer previously engaged in "bitter" primary battle

Per the Independent, both Onder and Schaefer served two terms in Missouri's state senate before bowing out after an unsuccessful bid for higher office, with Onder failing to challenge Luetkemeyer for the congressional seat in 2008 and Schaefer losing a 2016 primary challenge to be the state's attorney general against future Sen. Josh Hawley (R-MO).

According to St. Louis Public Radio, the 2008 contest between Onder and Luetkemeyer was exceptionally "bitter" and involved both candidates spending hundreds of thousands of dollars of their own wealth on that primary election.

Meanwhile, Luetkemeyer and Schaefer have been close friends for years and even jointly celebrated their respective 2008 victories.

Regardless of who wins the upcoming primary election, the 3rd Congressional District will almost certainly remain in the "R" column as the Cook Political Report rates the "open" district as "solid Republican" with an "R+16" advantage over the eventual Democratic contender.