Republican insiders in Pennsylvania have been waiting for a big show of unity around gubernatorial nominee Doug Mastriano since he won a harshly contested primary, and on Friday, they’ll finally get it.

Mr. Mastriano’s campaign — to this point quiet on the airwaves, out-fundraised and thriving mostly on word-of-mouth on the trail and through social media — will get a high profile, national boost from Florida Gov. Ron DeSantis, an increasingly popular figure in the GOP who is seen by party stalwarts as a model 2024 presidential candidate.

At the Downtown Wyndham in the late afternoon, Republicans will see Mr. Mastriano, the state senator, appear jointly at a rally with Mr. DeSantis, who’s making the trip as part of a swing to help Donald Trump-backed candidates in key electoral areas. After his Pittsburgh visit, Mr. DeSantis will head to Ohio to stump for Senate candidate J.D. Vance.

Democrats aren’t sweating the visit, and note that their candidate, Attorney General Josh Shapiro, has a sizable lead in early polling. They’re continuing to label Mr. Mastriano an extremist — chatter that has spread to Mr. DeSantis’ home state, where faith leaders and officials on Thursday warned that if their governor associates himself with Mr. Mastriano, he’ll do irreparable damage to his reputation.