KIEV, U.S.S.R.--Brimming with hope, Ukrainians voted yesterday in a referendum on independence that could deal the fatal blow to Mikhail S. Gorbachev's disintegrating Soviet Union.

All six candidates in an accompanying presidential election, including front-runner Leonid Kravchuk, have said the referendum will lead to full independence for the Ukraine and pledged not to sign Gorbachev's Union Treaty.

That appears to mean the Ukraine will break completely from Gorbachev's central Soviet government, unlike a few other republics which declared independence but signed the treaty.

But it was not clear whether the Ukraine could legally secede from the Soviet Union without recognition by the national legislature or Gorbachev, the Soviet president.

And there would be formidable logistical challenges to independence: the republic would have to set up its own border posts, enact its own citizenship laws and complete talks on removal of an estimated 1.2 million Soviet soldiers. The Ukraine's Parliament already has voted to create its own army and currency.

Irina Uspenskaya, 60, a retired economist and ethnic Russian, acknowledged the difficulties but said, "I'm voting for my grandsons. For me, I don't think anything is going to change soon, but for them, yes, I want independence."

Early turnout was reported at 75.6 percent. Preliminary results from the voting were not expected until this afternoon but pre-election polls indicated the Ukraine's 37.5 million voters were 3-1 in favor of independence because of nationalism and economic discontent.

Voters said Gorbachev had failed to raise their living standard and suggested the industrial and agricultural powerhouse--dominated for centuries by Poles, Nazis, Russian czars and Soviets--would be more prosperous on its own.

"Independence is the only way out of the current situation. It's easier to put order into small yard than a large yard," said Viktor Zakerpikny, 43, as he cast his ballot in Byelo Tserkov, 55 miles south of Kiev.

In Kiev, Mihailo Avanesov, 37, a metal artisan, said as he waited to vote: "If the center no longer takes a big slice of Ukraine's pie, we'll live fine."

The only early returns were from six Soviet military bases in Kiev, Odessa, Dnepropetrovsk and Donet- sk. The referendum carried there easily, with support ranging from 80.4 percent in Dnepropetrovsk to 97 percent in Kiev, said military spokesperson Vladimir Korkodim.

Soldiers at five of the six bases supported Kravchuk for president, giving him 49.7 percent to 70 percent of their votes. His closest challenger, Vyacheslav Chornovil, was leading only at the Kiev base with 44.6 percent of the votes.

The Ukrainian Parliament declared independence Aug. 24, and the referendum asked: "Do you support the act proclaiming independence of Ukraine?"

The Ukraine is so powerful economically and politically and has so many people--it is the second most populous republic with 52 million people--its secession would severely cripple Gorbachev as well as his government. Gorbachev has campaigned hard against any republics seceding, saying Saturday that Ukrainian independence would be a "catastrophe."

But he and his government have become increasingly irrelevant as the republics fill the political vacuum created by the failed hard-line coup against Gorbachev in August.

Aside from the Ukraine, another important republic was also asserting itself yesterday. Nursultan Nazarbayev, the president of Kazakhstan and one of the country's most powerful leaders, headed toward re-election as the sole candidate in the southern republic's first popular presidential vote.

Kazakhstan, with 16.5 million people, is the second biggest republic in area after Russia and one of the pillars of the Soviet Union's economic and military strength.

Back in Kiev, Kravchuk referred to the central government's vanishing role when he cast his ballot.

"In my conception, there is no center, only a coordinating organ that will regulate some relations"--such as control of nuclear weapons and cleanup of the 1986 Chernobyl nuclear disaster, said Kravchuk, who has pledged not to sign the Union Treaty.

The sincerity of Kravchuk, a former Communist Party boss, became the key campaign issue because he quit the party and embraced the independence drive only after the failed coup