Once allies, Mr Yushchenko and Ms Tymoshenko are now rivals
Ukraine's presidential election campaign is being formally launched, amid hopes it could drag the country out of its political paralysis.
Opinion polls suggest the pro-Western President, Viktor Yushchenko, is unlikely even to make a run-off in the election, set for 17 January.
Since coming to power after the Orange Revolution in 2004, political arguments have crippled his attempts at reform.
His popularity levels have dipped into single figures.
The main challengers for the role of president are expected to be Prime Minister Yulia Tymoshenko and opposition leader Viktor Yanukovych.
Former parliament speaker Arseniy Yatsenyuk is also expected to stand.
Ms Tymoshenko was once a close ally of Mr Yushchenko, and stood beside him during mass protests - the Orange Revolution - which followed the fraudulent election victory of pro-Moscow candidate Mr Yanukovych in October 2004.
When the vote was re-held in January 2005, Mr Yushchenko was elected, and appointed Ms Tymoshenko as prime minister. But the two soon fell out, and in September 2005 he dismissed her government.
With the Orange movement split, Mr Yanukovych staged a comeback, and was approved as prime minister.
But neither prime minister has been able to maintain a stable coalition, leaving the country hamstrung in the face of one of Europe's deepest recessions.
The economy is set to be a big issue in the presidential campaign. The currency has collapsed, and Ukraine has been forced to accept more than $10bn (£6.14bn) from the IMF, with tough conditions attached.
Among other campaign issues will be the future direction of the country. Mr Yushchenko has pursued an avowedly pro-Western foreign policy, with membership of Nato and the EU the eventual goal.
Mr Yanukovych draws strong support from Russian-speaking Ukrainians in the east and is likely to seek to improve ties with Moscow.