DURING THE past two years Sen. John Kerry (D-Mass) has emerged as the Obama administration’s key interlocutor with Syrian president Bashar al Assad. Now he is putting the dictator on notice that he has reached a make-or-break moment in his relationship with the United States.

Kerry has promoted the view that “engagement” between the United States and Syria could change the orientation of a regime that has been Iran’s closest Arab ally, and a weapons supplier to Hezbollah in Lebanon and Hamas in the Gaza Strip. As recently as last November, Kerry said after meeting Assad: “I remain absolutely convinced that there is an opportunity to have a different relationship with Syria.”

That was before Assad responded to mass protests in cities around his country beginning two weeks ago with brutal repression. So far more than 60 people have been slaughtered by his security forces for taking to the streets to shout slogans such as “we want only freedom” and “no to Iran.”

In an interview Tuesday, Kerry told me that he had contacted senior Syrian officials to demand an end to the killing. “I delivered as strong a message as I can that they have to avoid violence and listen to their people and respond,” he said. “Obviously the way the government has behaved is unacceptable. Sixty-one people killed is terrible, its abhorrant behavior.”

Now Kerry, like people across Syria, is waiting to hear a speech that Assad’s aides have promised he will deliver outlining a political liberalization in response to demonstrations across the country. “It’s a significant test,” Kerry said. “It’s a seminal moment.” The senator has heard promises of reform from the regime in the past. “I’ve always said, ‘put it to the test, don’t take it at face value,’ Kerry said. “You have to find out what people are prepared to do.”

Kerry indicated that he thinks Assad could still redeem himself with his people and with the United States. ”If he responds, if he moves to lift the emergency law, to provide a schedule for a precise set of reforms and a precise set of actions....we might begin to question whether something different is happening,” Kerry said.

In the meantime, the senator said he doesn’t favor aggressive action by the United States to bring the violence in Syria before the UN Security Council or seek sanctions, as was done when Libya’s Moammar Gaddafi began attacking his people last month. “I think it’s premature,” Kerry said. “You have to see what develops in the next hours. It could reach that point. I don’t think that with this fact pattern that is the choice to make.”

Trouble for Assad would seem to be a good thing for the United States, given his alliance with Iran and sponsorship of Hezbollah and Hamas. But Kerry says he worries about what could happen in a Syrian power vacuum. “Given Israel there are paramount considerations of what or what not might ensue,” he said. “There are a lot of question marks and they need to be profoundly thought through.”

Whether the U.S. would benefit from the downfall of Assad “depends on which Assad you are talking about,” Kerry said. “It depends on which direction the country is going in. It also depends on what the alternatives are.”

As Kerry sees it, we may soon see which Assad will emerge from Syria’s turmoil.