Syrian President Bashar al-Assad has a new American critic.

On Thursday, the chairman of the Senate Foreign Relations Committee, John Kerry, rapped Mr. Assad for not following through on his recent commitments to initiate political reforms.

Syrian officials have suggested in recent days that a national address Mr. Assad gave Wednesday would unveil sweeping new political changes for the Arab country. It didn’t.

“President Bashar al-Assad did not use his speech yesterday to promise concrete reforms, including lifting the emergency law,” Mr. Kerry said in a statement. “With large protests scheduled for tomorrow, it is essential that his government refrain from using violence against its own people.”

The Massachusetts Democrat has been perhaps Assad’s staunchest supporter in Washington. The two men have met a half-dozen times in recent years, according to Kerry aides, to discuss the Arab-Israeli peace process, Iraq and Lebanon. The two men have also mapped out specific ways to forge an agreement between Syria and Israel aimed at ending their dispute over the Golan Heights region.

Just two weeks ago, Mr. Kerry gave an address at the Carnegie Endowment for International Peace in Washington where he raised hopes of Mr. Assad playing a stabilizing role in the Middle East.

“President Assad has been very generous with me in terms of the discussions we have had,” Mr. Kerry said. “I think it’s incumbent on us to try to move that relationship forward in the same way.”

Mr. Kerry’s statement could presage a hardening Washington line toward Damascus as political protests are expected to continue in Syria. So far, the White House has taken a cautious approach. The U.S. has criticized the Syrian government’s use of forces against its people. But President Barack Obama’s administration hasn’t indicated it might pursue punitive measures against Syria, such as new economic sanctions, in response.