In the Wall Street Journal today, GOP strategist Karl Rove rejects conventional wisdom that Obama is vulnerable simply because of the two sided attacks from Clinton and McCain, but rather because in Tuesday night's Wisconsin speech he made a shift away from a centrist to a "left" political agenda. With Rove offering this new line of attack in the Wall Street Journal, expect it to be among the emerging talking points on talk radio and cable news.

Unlike Bill Clinton in 1992, Mr. Obama is completely unwilling to confront the left wing of the Democratic Party, no matter how outrageous its demands, no matter how out of touch it might be with the American people. And Tuesday night, in a key moment in this race, he dropped the pretense that his was a centrist agenda. His agenda is the agenda of the Democratic left.

In recent days, courtesy of Massachusetts Gov. Deval Patrick, Mr. Obama has invoked the Declaration of Independence, Martin Luther King, Jr., and Franklin Roosevelt to show the power of words. But there is a critical difference between Mr. Obama's rhetoric and that of Jefferson, King and FDR. In each instance, their words were used to advance large, specific purposes -- establishing a new nation based on inalienable rights; achieving equal rights and a color-blind society; giving people confidence to endure a Great Depression. For Mr. Obama, words are merely a means to hide a left-leaning agenda behind the cloak of centrist rhetoric. That garment has now been torn. As voters see what his agenda is, his opponents can now far more effectively question his authenticity, credibility, record and fitness to be leader of the free world.

The road to the presidency just got steeper for Barack Obama, and all because he pivoted on Tuesday night.