Samantha Power, the influential Obama adviser who called Hillary Clinton a "monster" in an unguarded moment, has resigned from the campaign.

"With deep regret, I am resigning from my role as an advisor the Obama campaign effective today. Last Monday, I made inexcusable remarks that are at marked variance from my oft-stated admiration for Senator Clinton and from the spirit, tenor, and purpose of the Obama campaign. And I extend my deepest apologies to Senator Clinton, Senator Obama, and the remarkable team I have worked with over these long 14 months."

Ms. Power, a brilliant, prodigal, Pulitzer-Prize winning author, was not asked to leave, campaign sources said.

Sparks like these show that tension between the campaign staffs is extreme.

I speak daily with aides, senior and junior, from both Clinton and Obama campaigns, and I can say, without revealing confidences, that the level of personal antipathy they express, the level of complete distrust, is extreme and in many ways alarming. One public example: when Obama's chief counsel, Bob Bauer, crashed a conference call held by Clinton advisers on Tuesday night.

The stress created by the interpersonal tension, and by the long hours, is taking a heavy toll. Many of the public faces you associate with your favorite campaign have worked 16 hours a day since January 3 with, maybe, three days off.

If Clinton wins the nomination, there will be many Obama staffers, particularly mid-to-high-ranking aides, who will refuse offers to help with the general election. The walk-away rate will be unprecedented.