President Viktor A. Yushchenko of Ukraine signed an order Thursday to dissolve Parliament and hold snap elections, after efforts to resuscitate a long-ailing pro-West coalition collapsed and sent the country deeper into political turmoil.

The vote will be on Dec. 7, a spokeswoman for the president said. It will be the third parliamentary election since Mr. Yushchenko came to power after the 2004 Orange Revolution, which promised a break from the former Soviet republic's corrupt, authoritarian past, but instead left Ukraine mired in political stagnation.

The political crisis could be compounded by global economic tumult and may undermine some of Mr. Yushchenko's policy goals, including NATO membership.

Mr. Yushchenko's decision followed weeks of raucous political infighting with his one-time coalition ally, Prime Minister Yulia V. Tymoshenko, whom the president has accused of orchestrating a power grab at the expense of Ukraine's national interests.

"I am convinced, deeply convinced, that the democratic coalition was destroyed by one thing - the ambition of one person," Mr. Yushchenko said in a television address broadcast on Wednesday during which he announced the dissolution of Parliament. "The Yulia Tymoshenko bloc has become the hostage of its leaders, who are willing to sacrifice everything: our language, security and our European prospects."

Ms. Tymoshenko, who recently sided with Ukraine's opposition to pass measures limiting the president's powers, is considering whether to contest Mr. Yushchenko's decision in Ukraine's Constitutional Court, a spokeswoman for the prime minister said. Her supporters have called for mass protests.

Some analysts have suggested that Mr. Yushchenko's decision to call elections was an attempt to undermine Ms. Tymoshenko ahead of presidential elections scheduled for January 2010.

"For Yushchenko to win a second term as president, his main task is to eliminate Tymoshenko," said Mikhail B. Pogrebinsky, director of the Kiev Center for Political and Conflict Studies.

He said the competition between the prime minister and the unpopular president was "complicating the stable development" of Ukraine.

Ms. Tymoshenko has increasingly presented herself as a centrist politician able to bridge the stark divide between pro-Western forces and those who want to maintain close ties to Russia.

She recently traveled to Moscow for talks with Prime Minister Vladimir Putin of Russia on energy cooperation, a meeting that was almost thwarted when Mr. Yushchenko requisitioned the prime minister's plane after his was said to have suffered a technical malfunction.

Ms. Tymoshenko's overtures to Russia have prompted criticism from Mr. Yushchenko's supporters that she has been co-opted by the Kremlin, although, according to Mr. Pogrebinsky, she will rely heavily on her pro-Western supporters in the December parliamentary election which he said her bloc is likely to win.

Meanwhile, the continuing political turmoil in Ukraine could jeopardize Mr. Yushchenko's bid to join NATO, a move that is far from popular among Ukrainians and many European NATO members, but strongly supported by the United States.

Speaking in Macedonia on Wednesday, Robert M. Gates, the United States defense secretary, reaffirmed Washington's support for Ukraine's NATO ambitions despite the political instability and said the United States would work with any new coalition that emerges.