Sen. Barack Obama is between a rock and a hard place.

Failure to hit back against the underhanded, negative politics of rival Sen. Hillary Clinton that resulted in Texas and Ohio defeats could leave him imperiled.

Yet if he does retaliate, his unifying message of hope and change from the bitter politics of the past may ring hollow with both supporters and undecideds.

He didn’t get this far - winning 26 of 40 contests against Clinton by our count - by having a weak stomach. He’s a fighter, too. But this is tricky business.

The Clintons are throwing everything at him to wrest away a nomination that, by all mathematical outcomes, is still Obama’s to lose.

He can’t let them, and if he’s smart, he’ll respond in kind.

Only Obama must accomplish this in a measured way that leaves his message and credibility intact, which is easier said than done.

Even with the shadow cast upon him by Tony Rezko, it’s hard to imagine Barack Obama losing any battle with Clinton on the subject of integrity.

The problem lies in the fallout from bringing up such topics.

He can’t be Barack Obama and stoop to that level. But if he can utilize some of his biggest political supporters to make the case for him, remaining just far enough above the fray to focus on his core campaign themes, it just might work.

The story of Obama’s posturing on NAFTA turned out to be bogus. Hillary is equally guilty of taking campaign funds from a con-artist (Norman Hsu).

The Clintons’ joint finances pose blatant conflicts of interest and the list of their connections to special interest groups and lobbyists sure isn’t short.

Negatives about Hillary Clinton matter to voters. Barack Obama’s efforts to run a clean campaign are admirable, but could, unthinkably, be his undoing.

A measured, but proactive response is vital. Soon.

With six weeks to go until the Pennsylvania primary, it’s time to fight back and make sure the nomination doesn’t slip through his fingers.