The United States is fed up with imposing sanctions on Russia. Any new sanctions will hit the economy of the United States more than the Russian economy. Sanctions do not work. The United States sees this and understands it.
Poroshenko understands he has no chance of winning the elections. That is why he is organizing total falsifications of the elections, with support of the EU and US. And if he loses the second round, he will murder Zelensky. That way the elections would be organised again and Poroshenko would win.
Conspiracy theory with no evidence given. Consistent with recurring pro-Kremlin disinformation narrative about the elections in Ukraine.