The party of Ukrainian President Viktor Yushchenko has formally pulled out of the ruling coalition, rejecting the prime minister's call to remain.
The party still has 10 days to reverse its decision. If it fails to do so, and there is no agreement on forming another coalition during the subsequent 30 days, Mr. Yushchenko has the right to call new parliamentary elections.
The president threatened snap elections Wednesday, after Prime Minister Yulia Tymoshenko's supporters joined the opposition to pass new laws weakening presidential powers.
Analysts say the political maneuvering is part of a political struggle ahead of the 2010 presidential elections.
Mr. Yushchenko and Ms. Tymoshenko were close allies during the 2004 Orange Revolution that brought the president to power. They have since disagreed, reconciled and fallen out again.
U.S. Vice President Dick Cheney is in Ukraine in a demonstration of support for the country's pro-Western leadership. He traveled from Tbilisi, where he criticized Russia's recent military action in Georgia.