"This administration has a feckless foreign policy which abandons American leadership," McCain told Fox News Sunday.
"What the conclusion you can draw is that this president wants to kick the can down the road on all of these issues until after the election. ... It's really an abdication of everything that America stands for and believes in," he later added.
Mitt Romney, Obama's presumed rival in this year's presidential election, said Obama "can no longer ignore calls from congressional leaders in both parties to take more assertive steps." He said the current approach has only given Assad more time to crack down on protesters.
"The United States should work with partners to organize and arm Syrian opposition groups so they can defend themselves," Romney said.
The White House called for Assad's ouster as recently as Saturday when it blamed the Syrian government for the deaths of more than 100 people, including 49 children, following peaceful protests. National Security Council spokeswoman Erin Pelton said the attack serves as a "vile testament to an illegitimate regime." The Syrian government has denied responsibility.
Earlier this month, at the meeting of leading industrial nations at Camp David, White House officials said they had hoped Russia could use some of its sway to halt the bloody crackdown and raised the possibility of modeling a regime change in Syria after Yemen. Yemen's longtime President Ali Abdullah Saleh stepped down in February as part of a U.S.-backed power transfer deal that gave him immunity from prosecution in return for relinquishing power.
According to U.S. Deputy National Security Adviser Ben Rhodes, Russian Prime Minister Dmitry Medvedev "did not dispute the fact that there needs to be a process of political transition" in Syria.