Elections unlikely to end long-running political standoff between president and PM.
But the two fell out and Yushchenko fired Tymoshenko’s government in 2005.
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Together, the former Orange Revolution partners could win enough seats to unseat Yanukovych.
“In one or two days we will announce the coalition,” Tymoshenko, smiling triumphantly, told reporters.
Yuriy Lutsenko, the leader of Yushchenko’s party, said it was ready to back Tymoshenko as prime minister after the coalition is formed.
A somber-looking Yanukovych made a brief statement in which he tried to present the results as his party’s victory, saying it would now start talks with potential coalition partners.
“They are trying to add 10,000 votes to the list – which is illegal,” said Woloshyn, an election monitor from the Canadian-Ukrainian Association.
After being accused of rigging the presidential vote in 2004, Yanukovych became prime minister after his party won a majority in elections in 2006.
Yanukovych, whose Regions party draws his support from Ukraine’s Russian-speaking east and south, fiercely resisted Yushchenko’s decision to dissolve parliament in April and call new elections after the president accused him of seeking to seize power.
Yanukovych eventually agreed to the vote on Sunday, but has suggested victory would be the only outcome he is prepared to accept.