President Trump is throwing his weight behind a fellow Republican's battle to keep an open House seat in GOP hands Thursday, in what could be a key bellwether for the November midterm elections.

Rick Saccone will face off against Democrat Conor Lamb on March 13 in a push to succeed former Republican Rep. Tim Murphy’s 18th Congressional District seat. Murphy resigned in October over a sex scandal in which his mistress claimed the pro-life conservative congressman pressured her to have an abortion.

“Will be going to Pennsylvania today in order to give my total support to RICK SACCONE, running for Congress in a Special Election (March 13). Rick is a great guy. We need more Republicans to continue our already successful agenda!” Trump tweeted.

Even though the seat will again be up in November, when Democrats hope to take over the House, hanging onto a seat on a district that voted for Trump in 2016 could stem Democratic momentum.

Saccone, a U.S. Air Force veteran, served in the Pennsylvania state legislature for nearly a decade. He is a pro-life candidate, who supports the repeal and replacement of ObamaCare, support for the military, and Second Amendment protections.

“I was Trump before Trump was Trump,” Saccone said in November. “I ran on that agenda in 2010. It’s the same agenda –it’s the people’s agenda. The president just nationalized it.”

While Saccone has seized on the Trump agenda, Lamb, a 33-year-old Marine veteran and former assistant U.S. attorney, doesn't mention Trump often and is focused on economic issues, healthcare and protecting Social Security and Medicare.

Fox News' Chad Pergram contributed to this report.