Michael Kinsley asks the question: "How have so many presidents gotten away with [skirting the Constitution's 'declaration of war' clause]?"
Then, some distance away from the question, he provides the answer -- unwittingly? I'm not sure. "[W]hy should members of Congress get to mouth off about war all day but not have to take shared responsibility for the yes-or-no decision?"
Precisely. John Boehner, Tom Price, John McCain, Jon Cornyn, Roy Blunt, Diana DeGette, Bruce Braley, Rand Paul and scores of others can snipe from the cloakrooms and score facetime on "Meet the Press" -- all without having to cast a definitive yea or nay. Of all its Constitutional prerogatives, actual decision-making isn't jealously ranked by Congress as one of its indispensable ones.
To restate what should be an unnecessary argument to all but the Constitutionally obtuse (who are, after all, hopeless), Barack Obama is doing nothing the Founders did not expect. They were all too familiar with executive power and the executive's outsized use of that power and thus they established a system of checks and balances; in cases of war, that Congress alone could declare it. They understood that American presidents would naturally gravitate to a self-concentration of power -- if it is to be checked, it is Congress' job to check it.
What they perhaps did not anticipate was that Congress would evolve as a mass of wholly self-interested, blathering reelection-experts who would happily abdicate their Constitutional responsibilities if such an abdication enhanced aforementioned reelection.
So let them blather. And let Mr. Kinsley ask his questions. For it is not President Obama, nor was it even George W. Bush or Lyndon Johnson or Harry Truman, who defined the "war powers" field. It was, and it remains, Congress.