LOWER MACUNGIE TOWNSHIP, Pennsylvania—Two years ago, working-class voters’ anger over the high cost of living helped carry Republican Ryan Mackenzie into Congress by a 1% margin. Now, those same pocketbook issues threaten to unseat the freshman incumbent, who is fighting for his political survival in a swing district. Mackenzie, 44 years old, hopes to persuade voters to give him another term by touting results he has delivered and attacking his Democratic opponent. He is also keeping close ties to President Trump, whose falling statewide approval ratings contrast with pockets of deep support in the 7th Congressional District, which Trump won…