President Obama's plan to leave up to 50,000 U.S. soldiers in Iraq following a 19-month withdrawal has some fellow Democrats saying he has abandoned his preelection position on the war.

But Obama's shifting from the 16-month withdrawal he urged while running for president isn't a dramatic change. And he always said the withdrawal should be fashioned to limit the loss of more American lives. His generals say the large residual force is needed to do that.

It isn't absolutely clear what all those troops will be doing. Obama said they would train and advise the Iraqi military. But what will be the response of these combat-ready soldiers if things start to fall apart?

That's the rub for those who voted for Obama as the antiwar candidate. They thought they had elected someone who would pull all 140,000 U.S. troops out of Iraq, no matter what. Blogger Justin Rainmondo of Antiwar.com now calls Obama a fraud.

Conversely, top Republicans, including Obama's former campaign foe Sen. John McCain (R., Ariz.), have hailed the president's withdrawal plan. "We are finally on a path to success," McCain said.

The Republicans were also heartened by assurances they said they had received from Obama to lengthen the timetable for withdrawal if conditions deteriorate. But that may not be so easy to do.

The Status of Forces Agreement signed by Iraq and the Bush administration requires all American forces to leave Iraq's "cities, villages, and towns" by the end of June. Obama said the U.S. combat mission in Iraq would end on Aug. 31, 2010, and the SOFA says all U.S. troops are to get out of Iraq by Dec. 31, 2011.

It's difficult to see how Obama could wiggle out of those deadlines without negotiating a new SOFA. And that seems highly unlikely, given the difficulty in getting the fractious Iraqi government to approve the document. Iraq's leaders are necessarily sensitive to criticism that they are America's puppets.

Then, there is that other issue that will play a huge role in whether Obama remains committed to his timetable to leave Iraq - Afghanistan.

Obama has ordered an additional 17,000 soldiers to Afghanistan, joining the 38,000 already there. He explained that the deployment was based on drawing down the number of soldiers in Iraq. What he didn't explain is the exact mission of the redeployed U.S. soldiers.

Sure, they're going to keep looking for Osama bin Laden, who launched the 9/11 attacks. But if they do find him, which now seems a remote possibility, could they then simply pack up and leave?

Taliban and al-Qaeda jihadists threaten not only the shaky Afghanistan government but also the nuclear-armed regime in neighboring Pakistan. Is Obama's objective to eradicate the jihadists from both South Asian nations? Or is it to achieve a more modest sense of stability that would allow a withdrawal, a la Iraq?

In both Afghanistan and Iraq, Obama is trying to steer a course that would provide enough troops to ensure their own safety but not deploy so many U.S. soldiers that he would be accused of wanting to occupy those countries indefinitely.

It's a tightrope act with many, many lives on the line. To get it right, Obama first needs to state clearly what he wants to achieve.