SVITLOVODSK, Ukraine — As two of Ukraine’s best-known investigative journalists, Sergii Leshchenko and Mustafa Nayyem showed a boundless zeal for exposing corruption and hypocrisy at the highest levels of government. So it set heads spinning within the country’s political and media elite last month when they suddenly announced that they were not only jumping the fence to run for Parliament, but also joining the establishment as candidates of President O. Poroshenko’s coalition party.

Or at least that’s how it seemed. In a sign of how hard it can be to kick old habits, Mr. Leshchenko and Mr. Nayyem have spent the final week of the campaign not working to promote themselves, but rather crusading to defeat a candidate supported by their own party — a former official in the Kiev city government, whose front-runner status in this rural district 250 miles south of the capital, they say, was the result of corrupt back-room dealing approved by someone close to Mr. Poroshenko, if not by the president himself.

“We want to demonstrate to society and to our political party at the same time that we are not going to tolerate this corrupt conspiracy,” Mr. Leshchenko said, sitting in the passenger seat of a white S.U.V. with a large red decal that said “Stop Corrupt Politicians” plastered on its front hood. Sounding genuinely torn, he added, “The most difficult part for us to accept was that we were going to fight against Poroshenko.”

That such infighting would start so fast and so furiously — even before Sunday’s vote — offers a glimpse of just how tumultuous politics remain in Ukraine eight months after the ouster of President Viktor F. Yanukovych, with a war against pro-Russian separatists still simmering in the east, hundreds of thousands of people displaced from their homes, Crimea annexed by Russia and an economy ever on the brink of collapse.