KIEV (Reuters) - Ukrainian opposition leader Yulia Tymoshenko launched her bid for the presidency on Tuesday as the early frontrunner in opinion polls, pledging to fight corruption, increase wages and speed up integration with the West.

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A fiery orator, she immediately emerges as a favorite in the March 31 vote to unseat President Petro Poroshenko, who swept to power in the wake of a 2014 revolution but whose popularity has eroded over his failure to stamp out corruption.

“A great country is my goal, from which I will not back down a single step. That is why I am running for president today,” Tymoshenko told party supporters at sports hall in Kiev, to cheers of “Yulia is our president”.

A former prime minister with a following among Ukrainian nationalists, Tymoshenko was jailed under pro-Russian leader Viktor Yanukovich, who defeated her to become president in 2010 and was toppled four years later in a popular revolt.

Released from jail, she again lost the presidency to Poroshenko, a confectionary billionaire who argued that Tymoshenko carried too much baggage to lead the nation.

While she retains a strong core of supporters, she may struggle to win over a majority in a country where many voters consider her a polarizing figure.

An opinion poll late last month by the Ilko Kucheriv Democratic Initiatives Foundation and the Kiev-based Razumkov think-tank put Tymoshenko in the lead, with support among likely voters of 16 percent. Poroshenko came second with 13.8 percent.

Tymoshenko said her foreign policy priorities will be Ukraine’s integration into the EU and NATO, which will help the country resist Russian aggression: “We must become a member of NATO right away. We don’t have time to hesitate,” she said.

After Yanukovich was toppled, Moscow swiftly annexed Ukraine’s Crimea peninsula and supported a separatist uprising in eastern Ukraine. That conflict, still unresolved, has killed more than 10,000 people. Major fighting ended with a ceasefire in 2015 but deadly clashes still occur regularly.

Tymoshenko said her team would increase worldwide diplomatic pressure on Russia to stop Moscow’s support of the separatists.

She has made economic promises which her critics say are incompatible with Ukraine’s obligations under a lending agreement with the International Monetary Fund.

She promised to increase salaries and pensions 3.5-fold during her five-year presidency and said she would change central bank policy to provide cheap loans to small businesses and mortgages to workers.

Poroshenko had proven he lacks the political will to fight corruption, betraying the principles that brought Ukrainians to the streets during the 2013-2014 revolution, she said.

“Stopping corruption will not be a problem. We will do it. This is possible. You and I have the political will. I promise to become a nightmare for these guys who feed on corruption.”