In a ruling that removes much of the fog surrounding the referendum, the Constitutional Court rejected tough conditions set for the poll by the Congress of People's Deputies, the Soviet-era legislature that has sought to emasculate Mr Yeltsin and block free-market change.
The court chairman, Valery Zorkin, announced the decision as the first of Russian's 107 million voters, fishermen and sailors in the northern Kola peninsula, cast their ballots four days early before setting out to sea.
The ruling coincided with opinion polls showing a steady increase in support for Mr Yeltsin, who needs a clear mandate from voters to resist a sustained assault by conservatives who dominate the Congress in Moscow and local soviets, or councils, in Russia's 49 regions and 16 republics.
Mr Yeltsin also faces strong opposition from his Vice-President, Alexander Rutskoi, who has declared his own ambitions to be president and yesterday continued a barrage of attacks on free-market reform: 'These are promises, but they are leading to nothing but impovershiment.'
But it was Mr Yeltsin who seemed to have the greater political momentum yesterday as his supporters injected life into an otherwise lacklustre campaign with a march through central Moscow and a rock concert on the edge of Red Square. More than 15,000 people gathered next to St Basil's Cathedral to proclaim their support for Mr Yeltsin.
It was the first sign of popular enthusiasm for Sunday's poll, which has plunged the country's political elite into a frenzy of name-calling but caused barely a ripple in the lives of most ordinary Russians.
Voters will face four questions: Do they trust Mr Yeltsin?; Do they support his economic reforms?; Do they want early elections for the President; Do they want the same for Congress?.
According to Yegor Gaidar, forced out of office as prime minister by conservative legislators in December, the real issue is starker. 'We shall decide one simple question,' he told Komsomolskaya Pravda. 'The question is whether we shall all face the firing squad.'
To reduce Mr Yeltsin's chance of victory, the Congress of People's Deputies had stipulated that he must secure support from not merely half the turn-out but half the entire electorate. Such a high victory threshold would have forced Mr Yeltsin to improve on his performance at the peak of his popularity two years ago, when he won a presidential race with 57 per cent of votes cast but only 40 per cent of the entire electorate.
Under new rules set yesterday by the Constitutional Court, Mr Yeltsin now needs only a majority of the turn-out to win a vote of confidence in his presidency and his reform programme. The only condition is that at least half of all registered voters go to the polls.
'I have no doubt that on the first question the President's victory is 100 per cent certain,' said a deputy prime minister, Anatoly Chubais, the architect of Russia's privatisation programme and one of several gung-ho reformers who will lose their jobs if Mr Yeltsin does not prevail. 'On the second question there is much more than 50-50 chance for the President's victory.' Another deputy prime minister, Sergei Sharkhrai, was more sanguine: 'Clear-cut victories do not exist in politics, and this referendum will not produce a clear victory.'