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The city that may be Ukraine's line in the sand

Relatives and mourners of 18-year old Vadim Papura, a pro-Russian activist killed after he jumped off from the Odessa Trade Union building on May 2, 2014, attend his funeral four days later.
Veli Gurgah | Anadolu Agency | Getty Images

Violent clashes have become a daily occurrence in eastern Ukraine ahead of the country's May 25 presidential election, and pro-Russian forces have been winning there. However, the southwestern port city of Odessa may be the proverbial line in the sand between the two sides.

Fighting in the "Jewel of Ukraine" this month marked the first time major clashes in a city that has seen significant—and so far successful—. Pro-Ukraine rioters killed at least 42 of their opponents in fighting and a fire Odessa earlier this month.

Eastern Ukraine: What the referendum means
Eastern Ukraine: What the referendum means

Odessa is more than 425 miles (about 700 kilometers) from Donetsk in eastern Ukraine, where pro-Russian forces have commandeered whole cities and this weekend held ad hoc referendums on independence.

It's Ukraine's biggest Black Sea port, even larger than Sevastopol in Crimea, which Russia annexed in March. Odessa lies in a region that forms Ukraine's maritime coastline and is of utmost importance to it economically.

"Ukraine without Odessa is effectively a landlocked country," said Matthew Rojansky, director of the Kennan Institute.

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Alexander Kliment, director of Russia and Emerging Market research at Eurasia Group, said unrest into those southern regions of the country would substantially weaken the Ukrainian government's legitimacy and security, though he thinks things are not yet at that point.

"If anything, the Odessa events showed that pro-Russian forces might have a significantly harder time gaining grassroots support there than they did in the eastern regions, which are closer, geographically, economically and culturally, to the Russian orbit," Kliment said.

US Ambassador to Ukraine: Russia, more sanctions, global markets
US Ambassador to Ukraine: Russia, more sanctions, global markets

A Stratfor report last week highlighted the difference between unrest in the eastern parts of Ukraine and in Odessa, the scene of the 1905 mutiny by the crew of the battleship Potemkin against the czar.

"Odessa, whose historical significance resonates deeply in Russia and whose port is vital to Ukraine's economic livelihood, could be exploited by Moscow as the Russians look for opportunities to cripple Kiev," the report said.

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Stratfor analysts also point out that because of the resistance in Odessa on the part of pro-Ukrainian forces, there would be much more risk for Russia if it decided to destabilize Odessa.

—By CNBC's Dina Gusovsky