Rep. Tom MacArthur, alone among New Jersey Republicans, has embraced Donald Trump with a full bear hug. Maybe he thought that would work out for him politically, but it's shaping up to be a disaster. He's toast as a statewide candidate in Blue Jersey, where Republicans had hoped he might rise to the U.S. Senate or governor's mansion. And now he has fresh reason to worry that he could even lose his seat in Congress. That reason is Conor Lamb, the rookie Democrat from southwestern Pennsylvania who just stunned the political world by marching into deep red territory and grabbing...