While Hillary takes aim at Obama's buddy, Tony Rezko, the Obama people need to explore the dimensions of Hillary's relationship with con artist Norman Hsu, who donated almost a million dollars to her campaign, money he'd scammed from gullible investors.

Obama needs to enlist the likes of Ted Kennedy and his other supporters in making the case against Hillary.

After her victories in Texas and Ohio, Hillary is once again in play. Negatives about her matter. A full frontal assault on her integrity and on Bill's conflicts of interest (remember they have joint finances) will work and will stop the bleeding in the Obama campaign.

The Clintons would like nothing better than for Obama to remain passive, clinging to his desire to raise the tone of our politics.

If Sen. Clinton resorts to negative ads in the remain ing primary states, Obama needs to do so, too, always keeping himself off camera and using an announcer to articulate his message.

As long as his attacks center on Hillary's and Bill's financial dealings and their relationships with lobbyists and special-interest groups, he will remain on message and his attacks will be in sync with his general approach to the race.

But Obama must make no mistake - it's time to fight back.