A report in The New York Times raises questions about the popular theory that Yushchenko was poisoned during a dinner with the head of Ukraine's security services.
A toxicologist tells the Times that because Yushchenko's symptoms manifested the next day, he was most likely poisoned about two weeks before the Sept. 5 dinner.
Yushchenko has not cooperated with investigators. He does not trust them, his campaign manager tells the Times.
Yushchenko has said he wants a "professional" debate with Yanukovych in the form of "questions and answers and the accountability of both sides for their words." In the previous debate, six days before Nov. 21 runoff, the two candidates traded personal attacks.
After two weeks of street protests, Ukraine's Supreme Court annulled the runoff results because of massive fraud, stripped Yanukovych of his victory and ordered the revote on Dec. 26.
Meanwhile, a convoy of Yushchenko's supporters moved deeper into the hostile eastern strongholds of Prime Minister Viktor Yanukovych.
The convoy — dubbed the "friendship journey" — of some 50 cars draped with Yushchenko's orange campaign colors has encountered several roadblocks put up by Yanukovych supporters.
Carrying about 150 people, mostly artists and musicians, the convoy is traveling around this France-sized nation of 48 million trying to sow support for Yushchenko in mostly Russian-speaking eastern and southern regions where Yanukovych draws most of his backing.
Despite fears of violence, convoy coordinator Olga Khodovanets said activists were still set to travel to Yanukovych's hometown of Donetsk and "negotiate free passage to the city with Donetsk authorities."
On Sunday, organizers canceled their visit to the Crimean city of Sevastopol after Yanukovych supporters blocked the city's main square, damaged several cars decorated with Yushchenko's orange ribbons and slightly injured one woman, the Unian news agency reported.
In another sign of the low regard for Yushchenko in Ukraine's east, assailants hurled a firebomb at his campaign office in the city of Mariupil in the Donetsk region, a statement posted on Yushchenko party's Web site said. There were no injuries, but the office was damaged in an ensuing fire, the statement said.
Police are investigating, party officials said.
Yushchenko, accompanied by his children, greeted thousands of Orthodox faithful who gathered on Kiev's Independence Square Sunday to celebrate a date marking the death of one of their most revered saints, St. Nicholas.
"Soon Ukraine will have a new president ... soon we will live in a new, democratic Ukraine," Yushchenko told the orange-clad, cheering crowd. Opposition supporters decorated 27 orange plastic Christmas trees representing Ukraine's regions.