Ms Tymoshenko, a former primeminister and key rival of Ukraine's President Viktor Yanukovych, is serving a seven-year jail sentence for abuse of office and faces another trial on tax evasion charges.
In two stints as primeminister, she reversed one of Ukraine's more controversial privatisations and scrapped an opaque intermediary in the gas trade between Russia and Ukraine.
It is rooted in a flawed change to the constitution in 2004 that reduced the power of the president but stopped short of turning Ukraine into a parliamentary republic, fudging the responsibilities of president and primeminister.
Ukraine's Vice PrimeMinister Hrihoriy Nemyria, a member of Ms Tymoshenko's bloc, denied the party was trying to form a coalition with Mr Yanukovych's party.