Yulia Tymoshenko alleged during an interview with The Associated Press that Poroshenko has used the position to enrich himself. She also alleged the president's associates have engaged in a sprawling effort to bribe Ukraine's voters to ensure his re-election.
Tymoshenko, 58, claimed voters were being offered 1,000 hryvnia ($37) in exchange for supporting the 53-year-old Poroshenko in the March 31 election. She said she asked the Ukrainian Interior Ministry to investigate.
"They are setting up that network of bribery across the entire country," she said. "I hope that the Interior Ministry will not allow that to be the basis of the president's campaign. How can a 'democratic' president treat his nation like that?"
Poroshenko's office quickly dismissed Tymoshenko's accusations. "Tymoshenko has been invariably leading the ratings of liars, and she obviously tries now to strengthen her positions," Poroshenko's press service said in a written statement in response to AP requests for comment.
The press service sought to turn the tables on Tymoshenko, charging that it was her political movement that engaged in bribing voters. Its statement noted that a movement activist was recently convicted of that crime and given a 5 ½ -year prison sentence.
Recent opinion polls have shown a 41-year-old comedian, Volodymyr Zelenskiy, who played the nation's president in a popular TV series, surging ahead of both Tymoshenko and Poroshenko in the presidential context.
A survey conducted jointly by four respected polling agencies that was released Monday had 21.9 percent of respondents supporting Zelenskiy while Tymoshenko had 19.2 percent and Poroshenko was third with 14.8 Percent. Other candidates trailed behind.
The poll of 10,000 people conducted in face-to-face interviews was completed last week and had a margin of error of 1 percentage point. Zelenskiy's high rating reflects both his popularity as a widely recognizable TV persona and the public disillusionment with current leaders.
Ukraine has been hit by economic troubles and a sharp plunge in living standards after Russia's annexation of the Crimean Peninsula and a separatist insurgency in the east. Tymoshenko served as the country's prime minister in 2007-2010. She later spent 2 ½ years in prison for striking a gas deal with Russia, which was largely viewed as retribution by her political rival, then-President Viktor Yanukovych.
She narrowly lost to Yanukovych in the 2010 presidential vote and to Poroshenko in 2014 after Yanukovych was driven from power by massive protests. In the AP interview, Tymoshenko pointed at Poroshenko's personal fortune and alleged the president, a multimillionaire with assets including a chocolate maker, media and other businesses, has profited from the fighting in eastern Ukraine.
"The president has made some of those earnings by lending his own money to the country at fantastically high interest rates during the war," she said. "This is an example of a conflict of interests. This is an example of how people abuse their senior position to engage in politicized, corrupt business."
In 2014, Forbes estimated Poroshenko's fortune at $1.3 billion. The value of his assets has shrunk since then. Tymoshenko promised to track down "every copeck" and show "how the country has been robbed during the war under the cover of patriotic slogans" if she wins the vote.
She also promised to initiate constitutional amendments to increase the powers of parliament. Tymoshenko, a native of the Russian-speaking east of Ukraine, has positioned herself as a pro-NATO, pro-European Union candidate and a staunch supporter of the troops fighting Russia-backed separatists in the industrial Donbass region.