WASHINGTON (CNN) -- As the Senate prepares to vote on a measure that would cut off funding in Iraq, a fascinating 2008 political subplot is emerging.

Democrats running for president are in a tough position on how they will vote.

Democratic leadership sources concede Senate Majority Leader Harry Reid's decision to hold a vote on an amendment that could cut off most funding for the war by March 31, 2008, puts Democratic presidential candidates in the Senate in a hard spot. All have been trying to appeal to the powerful anti-war Democratic base with various proposals, like repealing authorization for the war. But Sens. Hillary Clinton of New York, Joe Biden of Delaware, and Barack Obama of Illinois, have stopped short of wanting to choke funds to end the war.

Obama released a statement mid-afternoon Tuesday saying he will vote for the measure in order to "to send a strong statement to the Iraqi government, the president and my Republican colleagues that it's long past time to change course."

A few hours later, Clinton issued a statement saying she too will vote for the funding cut because the Democrats "must work together with clarity of purpose and mission to begin bringing our troops home to end this war."

Aides to Biden tell CNN they are not yet prepared to say whether they'll vote "yes" or "no" on an amendment to cut off funding for the war.

Those three Democrats' decisions on this vote are likely affected by a fourth Democratic presidential contender in the Senate, Chris Dodd of Connecticut. He is taking the unusual step of airing television ads beginning today in Iowa and New Hampshire, pressuring his colleagues to vote yes on cutting of funds.

"Half measures won't stop this president from continuing our involvement in Iraq's civil war," Dodd says in the ad. "That's why I'm fighting for the only responsible measure in Congress that would take away the President's blank check and set a timetable to bring our troops home. Unfortunately, my colleagues running for President have not joined me."

Clinton spokesman Philippe Reines told CNN "she's committed to finding ways to get the president to change course. That is what she has been working to achieve for years now."