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But Mr. Lee warned against reading too much into Mr. Altmire’s support among union households, saying that union voters are using the endorsements simply as one of several pieces of information while evaluating the candidates.

“I don’t think an Altmire win means a repudiation of union endorsement,” he said. “I think it means you can win despite it.”

“Altmire certainly looks stronger in the polls by all the internal measures of the survey. He’s got higher intensity. Everything seems to be breaking for him in the counties where voters make up the majority. It’s just, everything points to an Altmire victory.”

Still, Mr. Altmire’s 4 percentage-point lead is within the poll’s 4.9 point margin of error, showing the race is too close to call.

The winner will face Republican Keith Rothfus in a November general election rematch. Mr. Altmire beat Mr. Rothfus by less than 2 percentage points in 2010 for the 4th Congressional District seat.