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“No one will ever manage to force us into making an illegal decision, even if such indecent and unacceptable methods as threats and intimidation are used,” the commissioners said, according to the Interfax-Ukraine news agency.

Mrs. Tymoshenko and Mr. Yushchenko were leading figures in Ukraine’s 2004-05 Orange Revolution, which overturned a fraud-ridden presidential election originally won by Mr. Yanukovych. The pro-Western reformers received strong support from the United States and the European Union.

Feuding and infighting among Ukraine’s Western-oriented parties allowed Mr. Yanukovych to stage a political comeback last year, reclaiming the prime minister’s post last August. But the government has faltered in recent months, with neither side enjoying a clear majority.

Polls suggest that the Sept. 30 vote may not solve the crisis, with Mrs. Tymoshenko and Mr. Yanukovych claiming about a third of the vote and Mr. Yushchenko and other smaller parties sharing the rest.

Mr. Nemyria said Mr. Yanukovych’s party also could be trying to delay the parliamentary vote until later in the fall, moving them closer to the Russian State Duma elections scheduled for December.