Elections this month have enhanced the political clout of two groups widely separated on the political spectrum. The tea party movement stands to play an outsize role in the fall elections now that outsider Rand Paul has swept Kentucky's GOP Senate primary, while unions provided the muscle for Democrats to win a key special election in Pennsylvania.

Dr. Paul's victory comes just after Utah Sen. Bob Bennett was denied a place on the primary ballot by a GOP state convention dominated by tea party activists. In Kentucky, Dr....