Summary: Why does that the line from Yeats apply to America in our times? "The best lack all conviction, while the worst / Are filled with a passionate intensity."
One important reason is that the battle playing out in our politics is fundamentally a moral and spiritual battle, and while the right is connected to their moral and spiritual passions (even though that connection has been made on the basis of lies) Liberal America is not.
Much of that disconnection in Liberal America is due misguided beliefs, including: 1) that "value" is not really real, and 2) that there is nothing in the dynamics of the human world that warrants being called "evil," an "evil force," or "the battle between good and evil."
The crucial battle in America today is being fought in the political arena, but the heart of it goes deeper than politics. It is at the moral and spiritual level. The issue in America today is this: will constructive or destructive, life-serving or life-degrading forces prevail in shaping this nation's future?
I've written (here and "here") about President Obama's failure to match the force that has been attacking him relentlessly. But his failure is emblematic of the failure of Liberal America generally.
Why is there this mismatch between the people on the right, inflamed with an insistence on pressing the battle - an insistence that manifests itself, for example, in the repeal of Obamacare some fifty times - while their counterparts in Liberal America show little appetite for the battle?
The heart of the answer to this question about the mismatch lies in that place from which the "passionate intensity" must come in a battle of this sort. That place lies in the core of our humanity from which arise our moral and spiritual passions.
It was such passions that animated leaders in the rise of the Union against the over-reaching dominance of the Slave Power in the 1850s--leaders like Harriet Beecher Stowe and Abraham Lincoln. But so far, this time around, nothing has lit a fire in Liberal America like that lit by Uncle Tom's Cabin. And Liberal America has not yet raised up a leader like Abraham Lincoln, with moral and spiritual passion deeply integrated into his humanity, fortifying his resolve that, to keep the peace, there was so far he would bend but no further.
In America nowadays, most of the moral and spiritual passion is found on the other side, among those who have been seduced by the "worst" of America's spirits. Millions of people on the right have been set aflame with the determination to defend their sacred values. Unfortunately, their passionate intensity has been evoked and directed by lies. The picture of the world they have been sold is almost completely false, and the threats against which they have been mobilized to fight are bogus.
Nonetheless, the force on the right has recognized that power in our democracy can be gained through the "passionate intensity" of people who believe see themselves to be engaged in the battle of good against evil.
Much of Liberal America, by contrast, rejects the idea of there being any such thing as "the battle between good and evil." Those concepts are regarded as the residue of ancient religious ideas that have been rendered obsolete by the advance of a more modern rational and empirical ways of knowing. Committed to the values of intellectual integrity, requiring that conclusions be reached by applying reason to solid evidence, much of Liberal America -- certainly not all! -- lacks beliefs that provide a solid framework for moral and spiritual passion.
Even the reality of "value" is doubted. What's real is objective, some of our science-influenced philosophy has declared, and values are not to be found "out there" in our universe. Because values are "subjective," according to this view, they are but matters of opinion. Nothing can therefore be really good or evil.