Elections unlikely to end long-running political standoff between president and PM.
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Ukranians have faced three years of political paralysis as pro-Western Yushchenko has sought to push his Russian-backed rival from power.
After being accused of rigging the presidential vote in 2004, he became prime minister after his party won a majority in elections in 2006.
Yushchenko is pinning his hopes on a last-minute alliance with Tymoshenko – a partnership that could give their parties control of parliament, even if Yanukovych’s bloc wins the most votes on Sunday.
Forging a government coalition with Tymoshenko, however, could mean weeks of negotiation between their two parties, and Yanukovych has signalled that he would not give up power easily – threatening protracted election uncertainty.
Yanukovych, who 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.
Ukraine has 37.5 million registered voters but irregularities in voter registration has increased fears there could be legal challenges to any result.