President Donald Trump and a Republican candidate he backs hope to take a first step Tuesday toward flipping a Senate seat in swing state Pennsylvania.

The GOP faces a challenge in its push to unseat Democratic Sen. Bob Casey.

Rep. Lou Barletta, who endorsed Trump's presidential campaign in March 2016, will face state Rep. Jim Christiana in Tuesday's Pennsylvania Republican Senate primary. The winner will get to face Casey, one of 10 Democratic senators seeking re-election in states Trump won in 2016. Casey, running unopposed Tuesday, is considered one of the safer senators in the group, as the president only narrowly won Pennsylvania.

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