Ukraine's two biggest pro-Western opposition parties announced Monday they will be joining forces in the fall parliamentary election in order to challenge President Viktor Yanukovych's grip on power.

The parties led by jailed former Prime Minister Yulia Tymoshenko and former parliament speaker Arseniy Yatsenyuk said they will work together to reverse the democratic rollback that took place since Yanukovych came to power two years ago.

The alliance marks an attempt to break with years of infighting within the pro-Western camp that allowed the pro-Russia Yanukovych, whose fraud-tainted election victory was annulled in the 2004 Orange Revolution street protests, to make a comeback in the 2010 presidential election.

"We will form a democratic majority in the Verkhovna Rada (parliament), will limit Yanukovych's unlimited power and return the power to the Ukrainian people," Tymoshenko, 51, and Yatsenyuk, 37, who both ran against Yanukovych on a pro-Western platform, said in a joint statement. "There is no time to lose, it's time to fight and win together!"

Experts and opposition activists hailed the union with cautious optimism, saying it was high time that opposition leaders sacrificed their personal ambitions and united. But they noted that the union was fragile and did not include two other key opposition parties.

In the election, which is set for October, half of the 450 seats will go to deputies elected on party tickets, while the other half will be selected in individual races. Tymoshenko's party will take Yatsenyuk's votes in the proportional ballot, while the two parties will agree on individual candidates they will support. Four minor parties will also be part of the alliance.

During his presidency, the often Russian-leaning Yanukovych, 51 has moved to reverse the democratic achievements of the Orange Revolution. He has curbed anti-government protests, sent security forces to investigate civil society groups, tinkered with the Constitution to boost his powers and sought to limit press freedoms. He has also presided over the imprisonment of Tymoshenko, the country's top opposition leader and a key driving force behind the Orange Revolution.

The West has strongly condemned the jailing of Tymoshenko and several top allies as politically motivated and threatened to freeze ties with Ukraine.

Vadim Karasyov, a political analyst with links to the government, said uniting was the only way the opposition could survive.

"It's better this way than to be eaten one by one," Karasyov said. "The fewer opposition parties there are, the bigger a chance they have to succeed in limiting Yanukovych's power."

But political expert Volodymyr Fesenko pointed out that a pro-Western opposition party led by WBC champion Vitali Klitschko as well as a smaller party led by the respected Former Defense Minister Anatoliy Hrytsenko would run on their own and thus split the pro-democracy vote. Fesenko also said the Tymoshenko-Yatsenyuk alliance was shaky and could easily collapse before the vote.

"There is a lot of disagreement within the opposition and the chance of a split is real," Fesenko said.

Even if Tymoshenko's and Yatsenyuk's parties, who each have about 14 and 10 percent support respectively according to recent polls, do stick together, they are unlikely to win a majority of seats in the Verkhovna Rada, experts say. While Yanukovych's party of Regions enjoys some 20 percent support, its members are expected to dominate the single-member races due to their wealth and political connections.