A top Republican lawmaker called President Trump’s response to a suspected chemical attack in Syria a “defining moment” as commander in chief.

“They see our determination waning, but President Trump can reset the table here,” Sen. Lindsey Graham, R-S.C. said on ABC’s “This Week.”

Earlier Sunday, Trump condemned the reported attack in the Syrian city of Douma, blaming Syrian President Bashar Assad and his allies Russia and Iran.

According to Syrian opposition activists, a poison gas attack in Douma, which is held by rebels and near the capital of Damascus, killed at least 40 people. Syria and Russia denied the Assad regime was behind any chemical weapons attack.

"He has challenged Assad in the past not to use chemical weapons," said Graham, later adding, “If it becomes a tweet without meaning then he's hurt himself in North Korea, if he doesn't follow through and live up to that tweet, he's going to look weak in the eyes of and Russia and Iran.”

In Sunday morning tweets, Trump said there will be a “big price to pay” for the attack, and also blamed former President Barack Obama for his response to Syria’s use of chemical weapons in 2013.

“So this is [a] defining moment, Mr. President,” Graham said. “You need to follow through that tweet - show resolve that Obama never did to get this right.”

In a separate interview on ABC, White House homeland security adviser Tom Bossert said nothing should be taken "off the table” following the Syria attack.