Most in the Syrian opposition rule out negotiations with Assad, even on the terms of his departure from office.
Last week, U.S. Secretary of State John Kerry affirmed his support for the plan, saying that only a transitional government accepted by both the opposition and the Assad government can allow Syrians to determine their future.
The leader of the Syrian opposition coalition, Mouaz al-Khatib, has also suggested he opposes the formation of a rival Syrian government, saying he feared it would deepen divisions in Syria.
Breaking with opposition consensus, the 52-year-old former preacher provoked a backlash last month when he offered to hold talks with members of the regime if it would help end the bloodshed.
The formation of the interim government was put off twice over such disagreements, but Saleh said coalition members voted last month to go ahead with the election. Al-Khatib, while still opposed, is deferring to the majority, Saleh said.
Analyst Fawaz A. Gerges said that the move is likely to block a political solution.
"By electing an interim Cabinet, the Syrian opposition will put an end to any possibility for a negotiated settlement with the Syrian regime," said Gerges, director of the Middle East Center at the London School of Economics. "They've decided to fight all the way."
With an interim government in place, the "war option would win over diplomacy," Gerges said.
It's not clear where an interim government would be able to operate.
The regime routinely attacks rebel strongholds with airstrikes and artillery, and any gathering of senior opposition politicians would be a prime target. More likely, government members would shuttle between Turkey and Syria, as some rebel military chiefs do.
Acceptance is another challenge.
In recent months, Islamic extremist militias, particularly the al-Qaida-linked Jabhat al-Nusra, have asserted dominance in key battle areas.
Al-Nusra and other Islamic extremist fighting groups don't recognize the authority of the Free Syrian Army and might not be inclined to take orders from an interim government.
Saleh played that down, saying that 85 percent of the fighting forces recognize the Syrian National Coalition. Once the government moves into Syria and starts providing services, "doubts will just vanish," he said.
Landis predicted that the interim government would face a rough start. Trying to assert authority "is a recipe for conflict, no doubt about it," he said, "but they've got to get down to the towns and offer a real alternative."
Associated Press writers Barbara Surk in Beirut and Albert Aji in Damascus, Syria, contributed.