Democrat Patrick Slattery says he's no longer vying for the 134th House seat that opponent Republican in a special election in May.
Slattery posted on Facebook that he has withdrawn from the race due to personal and professional constraints.
Slattery and Mackenzie squared off in a special election to see who would fill the unexpired term of former Rep. Doug Reichley, who left state politics to become a Lehigh County judge.
The two men were set to compete again in November's general election for a two-year term for the same seat.
Though he had heard "rumblings," Mackenzie said he learned of Slattery's decision while going door-to-door in Emmaus on Monday to meet voters. "Didn't you hear?" he said a constituent, who had been watching the local TV news, told him. "Your opponent dropped out of the race."
But Mackenzie said he intends to continue to go door-to-door to meet voters nonetheless. He had planned to be knocking on doors in Lower Macungie on Tuesday evening. "It's the best way to get out your message," he said, "and to represent the people of our district."
Of Slattery, Mackenzie said, "I wish him well. He probably just didn't have the fight in him for a third time." (Slattery also had run for the seat in 2010 against Doug Reichley).
Mackenzie said it's possible he will face another contender if the Democratic Party names a candidate by early August to replace Slattery on the ballot.