In 1998, Charles Schumer bested three-term Republican incumbent Al D’Amato in New York’s Senate election, and since then, he rose to become chairman of the Democratic Senatorial Campaign Committee for the 2006 and 2008 elections. He is also the third-ranking Democrat in the Senate and chairman of the Senate Democratic Policy and Communication Center. A friend to Wall Street—an important constituency in New York—Mr. Schumer has split with the party line on top-bracket tax rates. Ever the proud New Yorker, Mr. Schumer has found his way into local issues that other senators might not bother with. He managed to work Jeremy Lin’s name into every speech during Mr. Lin’s 26-game Knicks career; he spearheaded efforts to ban inhalable caffeine, Four Loko energy booze, anonymous prepaid cellphones (because terrorists can use them) and synthetic marijuana. Mr. Schumer is a hero to Democrats and a bane to Republicans. Either way, he is at the heart of the big issues, and everyone in Washington must reckon with New York’s feisty, Brooklyn-born senator.