Former President Donald J. Trump on Saturday endorsed Doug Mastriano, a retired colonel and state senator who has propagated myriad false claims about the 2020 election and attended the protest leading up to the Capitol riot, in the Republican primary race for governor of Pennsylvania.

Mr. Trump made his choice three days before the state’s Tuesday primary, a political blessing that serves to increase the former president’s standing as much as Mr. Mastriano’s.

“There is no one in Pennsylvania who has done more, or fought harder, for election integrity,” Mr. Trump said in a statement, adding that Mr. Mastriano would also “fight violent crime, strengthen our borders, protect life, defend our under-siege Second Amendment, and help our military and our vets.”

A Fox News poll released Tuesday showed Mr. Mastriano with a lead of 12 percentage points over his closest primary rival, former Representative Lou Barletta.

Since then, Mr. Barletta has sought to coalesce support from Republicans wary of nominating Mr. Mastriano. Two fellow candidates dropped out and endorsed Mr. Barletta, as have a few prominent former elected officials, including former Senator Rick Santorum of Pennsylvania.

Mr. Trump, whose chosen candidate for governor of Nebraska lost a primary on Tuesday, is at risk of another blemish on his record in Pennsylvania’s Senate race. His pick, the television personality Dr. Mehmet Oz, has failed to put daylight between himself and a field of candidates.

Mr. Mastriano has long been an outspoken supporter of Mr. Trump. He used campaign money to organize buses to Washington on Jan. 6, 2021, and, last month, campaigned at an event that promoted the outlandish QAnon conspiracy theory.

Pennsylvania Republicans not aligned with the Mastriano campaign have said he cannot win a general election against Josh Shapiro, the Pennsylvania attorney general who is the presumptive Democratic nominee for governor. Mr. Shapiro’s campaign recently began airing television advertisements that appeared intended to lift Mr. Mastriano’s standing among Republican primary voters.