Harry Reid effectively owns the U.S. Senate. Democrats and Republicans have long debated how Reid managed to get such a tight grip on the upper chamber. But there is no debating this much: Nothing happens on the floor, or in committee, without the Nevada Democrat's approval. Most important Senate legislation is written in Reid’s office and sent directly to the floor for a vote on final passage, sidelining committee chairmen who historically have functioned as an influential and quasi-independent force. In the past, a bill would not go to the floor before a series of votes and amendments in whatever...