Mitt Romney did much more this weekend than announce a running mate. He unveiled a significant change in strategy. The 2012 election is now a choice, not just a referendum.

Conservatives have spent much of this summer reassuring themselves. They've pointed out the extraordinary sums President Obama has thrown at crippling Mr. Romney. They've noted how ugly and brutal those attacks have been. They've comforted themselves that, for all the smears, Mr. Romney is within a few points of the incumbent in national tracking polls....