Once allies, Mr Yushchenko and Ms Tymoshenko are now rivals.
Ukraine's parliament has set a date for the country's presidential election, ending a dispute over its timing.
It will be held on 17 January, after a motion proposing that date was approved by 399 out of a total of 450 lawmakers.
The vote had been scheduled for October 2009, but President Viktor Yushchenko challenged that date and received the backing of the Constitutional Court.
Prime Minister Yulia Tymoshenko, his political rival and former ally, says she also intends to run for president.
Mr Yushchenko, who took power after the so-called Orange Revolution of 2004, has seen his popularity plummet since taking power.
Presidential polls in Ukraine have traditionally been held on the last Sunday in October - but Mr Yushchenko took power in January 2005, three months after mass protests followed the fraudulent election victory of pro-Moscow candidate Viktor Yanukovych.
And in May 2009, the Constitutional Court - Ukraine's highest court - agreed with the president's assertion that an October election would be "unconstitutional", overruling the date originally set by parliament.
But the BBC's Gabriel Gatehouse in Kiev said three months of extra campaigning might not be enough to reverse the president's rock-bottom popularity rating.
He added that the rivalry between Mr Yushchenko and Ms Tymoshenko had paralysed the workings of government.