"It has taken President Obama far too long to speak out forcefully against Assad and his vicious crackdown in Syria," the former Massachusetts governer said in a statement. "In the early stages of this crisis, the Obama Administration referred to Assad as a 'reformer,' which had the effect of emboldening Assad and discouraging the dissidents. America must show leadership on the world stage and work to move these developing nations toward modernity."
Secretary of State Hillary Clinton had cautioned against a rush to declare that Assad must go, saying such a move is more effective when others do so as well. To that end, the administration has been in talks with other countries to take coordinated action today as well.
The Obama administration has stepped carefully when discussing Syria. In his May 19 speech on this year's uprisings in the Middle East and North Africa, the president declared Assad must reform or get out of the way. The administration eventually said Assad has lost that opportunity to reform and Clinton reluctantly said last month he had lost his legitimacy.
Recently, the administration has gone to the brink of calling on Assad to step down, with White House spokesman Jay Carney and Ambassador to the U.N. Susan Rice both saying that Syria would be better off without Assad in power.
The State Department began to prepare last week for the eventual change in policy when spokeswoman Victoria Nuland said the administration's policy of engagement with Assad was over.
The U.S. has announced in recent weeks new sanctions on regime insiders and their business interests, hoping to cut off funding and support for Assad. It sent a senior adviser, Fred Hof, to Europe last week to rally support for coordinated action.
The administration is developing tough oil and gas sanctions, aiming for a key source of funding for the regime, and has urged other countries to halt energy deals with Syria. Clinton last week took to naming and shaming countries who still import energy from Syria and urging them to "get on the right side of history."
Clinton specifically named India and China and said Europe must do more to squeeze the regime. She also called out Russia for continuing arms sale to Syria.
ABC News' Devin Dwyer and Jim Sciutto contributed to this story, which was supplemented with Associated Press reporting.