Russians will go to the polls tomorrow to decide whether Mr Yeltsin will go on being their president, or whether to confer power on the bull-dog shoulders of Gennady Zyuganov, the leader of a Communist-nationalist bloc. After the events of the last few days, the choice will surely seem harder than ever before.
When they voted in the election's first round on 16 June, many of the 26 million who put a cross against Mr Yeltsin's name would have believed he was a rejuvenated man. This was one reason they were willing to set aside their grave reservations about his blundered handling of the Chechen war, a broken-down economy, and the wave of crime, corruption and social ills that has come hand-in-hand with the arrival of the free market.
Their 65-year-old president seemed to have bounced back like an India rubber ball, apparently having overcome both last year's heart trouble and his love of liquor. As the campaign unfolded, they watched him on state-controlled television touring the length and breadth of the nation.
On one day, he was down an Arctic coal mine; on another, he was dancing at a rock concert in the Urals; on a third, he was arguing enthusiastically with a gaggle of old women in the street. For all his many shortcomings, lack of energy and determination did not seem to be among them.
Thanks to this new lease of life, plus a well funded and professional campaign team, Mr Yeltsin came first with 35 per cent, some three points clear of Mr Zyuganov. The margin, though slight, was enough to justify cautious optimism, both in the Kremlin and in the West, that victory in the run-off would be his. Now, although he still seems likely to be re- elected - unless the turn-out falls steeply - the contest is much altered.
Yesterday, after four days of mysterious seclusion, Mr Yeltsin appeared on television, looking weary, pale and very wooden, to appeal to Russians to vote for a "normal life".
His younger rival, Mr Zyuganov, has been having a field day, demanding a medical commission to investigate the president's health, calling for a coalition government, and suggesting that Yeltsin is unfit to govern. An otherwise dull performer, the Communist leader has a new assurance about him these days, as he trots out his lines before the cameras in a near-monotone. (He has even borrowed his opponent's tactics, inviting journalists to watch him play volleyball, and dancing at a nightclub).
You could understand the concern of the president's handlers - who include his daughter, Tatyana - to get him on the air. In the last week, Mr Yeltsin has cancelled three campaign visits because of what his aides described as a sore throat. We do not know what his true condition is, but it seems certain to involve more than his vocal cords (which seemed to work during yesterday's television address). Although his health problems have been played down, or even ignored, by much of the Russian media, word has still got out - especially through the pages of the Communist-dominated press in the regions. On the eve of the most important election in the country's history, Russians needed to be shown that their president was still alive, if not particularly well.
On Sunday, he was supposed to stage his grand finale, an appearance at Moscow's Luzhniki stadium before thousands of cheering young people, lured there by the promise of rock bands and beer. He did not appear in person. That privilege was left to his puppet from a satirical television programme called Kukly, Russia's equivalent of Spitting Image.
It was an unfortunate choice of stand-in. For, as Russians prepare to vote, the election is dominated by serious doubts over whether Mr Yeltsin could turn out to be little more than a puppet, an ill old man manipulated by a coterie of ambitious insiders hungry for his job.
The stagnation of the latter years of Leonid Brezhnev's regime still lurks in the collective memory. Could Boris Yeltsin be heading for the same fate, and - if so - who would run the place?
The question mark over his health revives an issue that has been hanging in the air ever since Russia's constitution passed into law by referendum in 1993, in a vote that is now known to have been rigged. Mr Yeltsin's post became the most powerful elected office in the world, allowing him to rule by presidential decree, without paying much heed to the weak, now Communist-dominated, parliament. With so much power concentrated in one man's hands, the system depends on an active president - and not an ailing one.
When Mr Yeltsin had his first bout of heart trouble last July, it was widely believed in the West that his successor would be Viktor Chernomyrdin, Russia's prime minister. The former head of the state-run gas giant, Gasprom, was riding high in public opinion after successfully intervening in a crisis in the southern Russian town of Budyonnovsk, where Chechen rebels had taken taken more than 1,000 people hostage. But, after piloting the government's political party, "Our Home Is Russia", to a poor performance in December's parliamentary election, Mr Chernomydrin's star has waned. Few now see him as a likely heir, let alone the favourite.
For now, that slot is filled by the erect military frame of Alexander Lebed, the former two-star paratrooper general on whom the president has lately lavished power and glory. Two weeks ago, Mr Yeltsin appointed him secretary of the Security Council and his national security adviser in the hope of wooing the 11 million voters he attracted in the first round.
At first, it seemed a smart move, not least because the general's arrival in the Kremlin precipitated the firing of four unpopular hawks, none of whom are known for their love of democracy - including his arch-enemy, the Defence Minister, General Pavel Grachev, and the head of the powerful Federal Security Service, General Mikhail Barsukov. Since then, however, Mr Lebed's contribution has been far more questionable.
He has, for example, given some intriguing hints about how he sees his future role. He has announced that he wishes to be vice president (although this would involve a change in the constitution that, by law, should be ratified by both parliamentary chambers and two-thirds of the 89 regions).
He has also declared that he wants sweeping new powers for the Security Council, giving it oversight over the military and security services, and allowing it to carry out his mission of imposing law and order over a fledgling society that is riddled with corruption and organised crime.
If Mr Yeltsin is fit and active, Mr Lebed's powers - if he gets his way - would indeed be great. But if the president is closeted in a country sanatorium, surrounded by cardiologists, the general clearly aims to rule the roost.
His recent erratic behaviour (not least, calling Mormons "scum" and whipping up fears of a coup) has sent a frisson of alarm through Western governments. They know he just might end up being in charge of the largest country in the world, with a huge nuclear arsenal, a disaffected army, feuding security services, and uncertain legal and democratic structures.
But Mr Lebed may not survive for long. His behaviour has shown him to be politically naive, and unwise. Although popular in the army's lower ranks, he has plenty of strong enemies in the Kremlin, who know how to plant a knife in the back - and will not hestitate to do so. Once the election is over, Mr Yeltsin may also wonder if he has any further need of him.
If he goes, others will step forward in search of power. Mr Yeltsin's close confidant, General Alexander Korzhakov, whom he sacked a fortnight ago as the head of the presidential guard, is still working in the Kremlin in an unclear role.
Another close ally, the mayor of Moscow, Yuri Luzhkov, who was elected last month with an overwhelming majority, is rumoured to be seeking high position. And other heavyweights lurk in the wings, from ministers - the Foreign Minister, Yevgeny Primakov, the Interior Minister, Anatoly Kulikov - to liberal advisers, notably the former privatisation minister, Anatoly Chubais, and the president's top campaign co-ordinator, Sergei Filatov.
If Mr Yeltsin is re-elected, the future is unlikely to be an easy one. Russian politics is about personalities. In the last three months, Mr Yeltsin's campaign team proved that they could overcome his deep unpopularity - by whipping up a souffle of support, aided by the master-chefs of the Russian national media.
But, unfortunately, the choice of people is not inspiring. Tomorrow Russia will decide between a president with poor health, backed by a power-hungry general, and Gennady Zyuganov, an unknown apparatchik whose senior aides include some Soviet-era dinosaurs and a disturbing element of hardline nationalists. Russians have yet to get the politicians they deserve.