KIEV, Ukraine — Parliament elected the fiery Yulia Tymoshenko prime minister yesterday by the narrowest possible margin, in a striking political comeback likely to strengthen Ukraine’s ties to the West and aggravate tensions with Russia.

Now the big question is whether the 47-year-old heroine of the 2004 Orange Revolution, which split the country between those who favor close ties to Moscow and those who seek greater integration with Europe, can hang onto her job.

Yesterday marked the second time Ms. Tymoshenko has won the prime minister’s post. Her first stint ended after just seven months, when she was fired by her Orange Revolution partner, Viktor Yushchenko.

Speaking shortly before the vote, Ms. Tymoshenko said it was critical that the two parties put their differences aside.

“Today’s vote is a moment of truth for the democratic coalition,” she told Parliament.

Moscow openly endorsed the Orange Revolution’s major foe, Viktor Yanukovych, in the 2004 presidential contest — and the Kremlin has bitterly denounced the results as part of an effort by the West to weaken and surround Russia. But Moscow reacted with soft words yesterday, welcoming the prospect of a Cabinet taking shape in Ukraine.

Ms. Tymoshenko outraged the Kremlin in April, when the American magazine Foreign Affairs published an article in which she urged Western nations to oppose what she called Moscow’s effort to restore control of its “lost empire.”

More recently, she vowed to get rid of a company, half-owned by Russia’s state-owned Gazprom, that acts as a middleman in Russian natural gas sales to Ukraine. She has called the sales arrangement “corrupt.”

“There cannot be any mediators on the gas market,” she said yesterday.

Ms. Tymoshenko received 226 votes — the minimum required for confirmation by the 450-member Parliament — from deputies in her bloc and Mr. Yushchenko’s party.

Ms,. Tymoshenko took the oath of office clad in a dazzling white dress and wearing her signature coif — her blond hair braided and curled in a halo.

“I congratulate everybody who voted for the democratic forces, and those who did not — we will make sure that we are their team too,” Ms. Tymoshenko said, with a triumphant smile. “What we have to do now is show society high-quality results.”

But the narrow vote was an ominous sign of how difficult it will be for the Yushchenko-Tymoshenko coalition to govern.

The Party of Regions, led by the Orange Revolution’s old adversary, a former prime minister, Victor Yanukovych, is expected to challenge Ms. Tymoshenko and Mr. Yushchenko’s allies in Parliament.

Yanukovych demonstrated his skill at parliamentary politics last year by wooing his opponents in the legislature to join in a coalition with the Party of Regions.

Tymoshenko and Yushchenko have a history of tensions. Both have shown interest in running in the 2010 presidential elections.

Viktor Luhovyk, a political analyst with the Dragon Capital investment house, said that while the Orange leaders have agreed to share power for now, their partnership is unstable.