MOSCOW, April 2 ? A fragile power-sharing deal in Ukraine collapsed Monday when President Viktor A. Yushchenko ordered the dissolution of Parliament, the base of support for his rival, Prime Minister Viktor F. Yanukovich, whom the president accused of usurping power. Mr. Yushchenko ordered new elections for May 27, less than eight weeks away.

The maneuvers pushed Ukraine into its worst political crisis since the 2004 Orange Revolution, when Mr. Yushchenko defeated Mr. Yanukovich after street protests reversed a fraudulent election. The rivalry between Mr. Yushchenko, a pro-Western leader, and Mr. Yanukovich, who is supported by Russia, has been simmering in Ukrainian politics since then, reflecting a divide in Ukrainian society between a Russian-speaking eastern area and Ukrainian-speaking west.