Bernie Sanders campaigned, hard, for Barack Obama's reelection.
But the independent senator from Vermont is not going to rubber-stamp the president's selection of Jack Lew, a supporter of banking deregulation who has passed back and forth through the revolving door from Wall Street to Washington, as the nation's 76th Secretary of the Treasury.
While Sanders caucuses with the Democrats, he represents the people who elected him. And he swears an oath to a Constitution that requires -- not "allows," requires -- the legislative branch of the federal government to check and balance the executive branch.
One of the Senate's most vital duties is that of providing "advice and consent" on presidential nominations. A president has broad leeway when it comes to naming members of the Cabinet -- arguably broader leeway than in the naming of lifetime appointees to the federal judiciary. But that leeway is not such that senators can or should simply approve every nominee. Advice should be given, and at times consent should be denied -- not just by partisan foes of the sitting president but, sometimes, by allies of that president.
In the hyper-partisan environment of today's Washington, it is common for members of the party caucus affiliated with the president to go along with any pick the president makes. But there are times when principle must prevail over partisanship.