President Barack Obama vigorously defended the nuclear deal with Iran on Wednesday, casting the historic accord as the only possibility to avert a nuclear arms race in the Middle East and reduce the chances of war.

A day after the U.S., Iran and world powers announced the deal, Obama said the U.S. faces a "fundamental choice" about whether to embrace the opportunity to resolve the Iranian nuclear issue peacefully. His remarks in a White House news conference appeared aimed squarely at Congress, where lawmakers are discussing legislation to try to stop the deal's implementation.

"I expect the debate to be robust, and that's how it should be," Obama said, imploring lawmakers who are skeptical of the deal to "remember the alternative."

Under the deal announced Tuesday, Iran's nuclear program will be scaled back and closely monitored as the U.S. and world powers seek to cut off Iran's ability to develop a nuclear weapon. In exchange, Iran will see biting economic sanctions eventually lifted, freeing up billions of dollars in oil revenue and frozen assets.

Obama, taking questions from reporters in the East Room, said that in the absence of a deal, the international economic sanctions that brought Iran to the negotiating table will unravel, and the world community will be unable to put the sanctions regime together.

"Without a deal, we risk even more war in the Middle East, and other countries in the Middle East would feel compelled to develop their own nuclear weapons," Obama said, adding that such a chain of events would risk a nuclear arms race "in the most dangerous region in the world."

As Obama defended the deal, his allies were mounting a concerted push to sell the agreement to skeptics, while the deal's critics warned of dire consequences.

Vice President Joe Biden spent the morning on Capitol Hill briefing House Democrats, and told reporters he was confident that lawmakers would get behind the deal. Yet in Jerusalem, Israeli Prime Minister Benjamin Netanyahu deplored the agreement in remarks to parliament, warning that Israel would "reserve our right to defend ourselves against all of our enemies." In Tehran, Iranians took to the streets to celebrate the accord, and even Iran's hard-liners offered only mild criticism -- a far cry from the outspoken opposition that the White House had feared.

Opponents of the deal, including Israel, have lambasted the Obama administration for granting sanctions relief to Iran while it continues to fund terrorist groups in places like Syria, Yemen and Lebanon. Obama said the U.S. would keep trying to gain Tehran's cooperation on other security issues, but acknowledged the Islamic republic might not change its behavior.

"We're not betting on it," he said.

Aiming to frame the parameters for the growing debate, Obama bemoaned that his political opponents have wielded "speculation or misinformation" about the deal. While he said he hoped Congress would approve the deal based on the facts, he conceded that "we live in Washington, and politics do intrude."

"I am not betting on the Republican Party rallying behind this agreement," Obama said.