President Vladimir Putin and his advisers in the Kremlin are said to believe too much is at stake to allow the strategically important former Soviet republic to drift further into the western European orbit.
"Moscow is quite unhappy with the prospect of a Yushchenko victory and, from this point of view, it would like to work against him by any means," political analyst Yevgeny Volk, director of the US-based Heritage Foundation in Moscow, said.
"Russia's influence is quite limited but I would expect any kind of provocation against Yushchenko. The situation is very fragile because many people in the Kremlin have convinced Putin that this is a matter of life and death for Russia."
Putin's elaborate efforts to influence the outcome of the disputed election have embarrassingly failed, with opinion polls showing Yushchenko well ahead of pro-Russian candidate Prime Minister Viktor Yanukovich in the lead-up to Sunday's repeat run-off vote.
He paid two trips to Ukraine to boost Yanukovich's chances and publicly hailed his favourite's "victory" on the night of the first run-off last month, which was eventually annulled due to widespread fraud.
Cold War struggle
Analysts said they expected the Cold War-style struggle for influence in Ukraine to continue after Sunday's vote, even if the Kremlin is now resigned to a Yushchenko victory.
"The Kremlin is desperately trying to come up with a response," Sergei Markov, a Kremlin-connected political consultant who advised Yanukovich during his ill-fated campaign, said.
"Yushchenko is very intelligent but he's weak and the big question is who will wield influence behind the scenes, and that's what the Kremlin is working on."
"The situation is very fragile because many people in the Kremlin have convinced Putin that this is a matter of life and death for Russia"
Russia's political elite is believed to fear Ukraine eventually joining the US-led NATO military alliance and the European Union, further eroding Russia's standing.
It will also deal a major blow to Putin's unstated ambition of restoring Russian dominance over the former Soviet empire in eastern Europe.
Heritage Foundation's Volk said, "The problem is that Putin is a captive of Cold War stereotypes and his vision of the world is very black and white.
"[Putin] understands the world according to the 'zero sum game' whereby anything that is good for the West is bad for Russia, and vice versa."
Shock for Putin
Andrei Piontkovsky, a Moscow-based political analyst, likened the West to a "gallant cavalier trying to seduce" Ukraine.
"Russia behaves like an impotent rapist and this makes its position ridiculous. It's perfectly obvious in this situation whom the girl will choose," he said.
Viktor Yushchenko is expected
to win the forthcoming election
"The events in Ukraine were a huge shock not just for Putin but for the majority of the Russian political elite. For the first time they had to recognise that Ukraine is an independent state."
None the less, according to Volk, Ukraine's large Russian-speaking, generally pro-Moscow population in the east of the country, as well as its economic dependence on Russia, will give the Kremlin powerful leverage even after Yushchenko's expected victory.
"As usual they'll try to use economic leverage to make Yushchenko docile. Ukraine still depends on Russian oil and gas supplies, and imports of its agricultural products," he said.
Volk added, "But I don't believe that relations will worsen dramatically because Moscow and Kiev need each other."
Yushchenko will also be restrained by the fact that more than half of Ukraine's population of 48 million people are Russian speakers with strong cultural ties to Russia, analysts said.
"The events in Ukraine were a huge shock not just for Putin but for the majority of the Russian political elite. For the first time they had to recognise that Ukraine is an independent state"
After weeks of often heated denunciations of alleged Western meddling in Ukraine, Putin insisted last Friday that he would be "pleased" to see the country admitted to the European Union.
But he said that this was unlikely to happen anytime soon.
"If Ukraine wants to enter the EU and is welcomed there, then we can only be pleased," Putin said at the Kremlin, adding however that it was "not our business".