In this report:

• A 'vacuum of power'
• Yeltsin championed democratic reform
• Potential successors

MOSCOW (CNN) -- As Russian President Boris Yeltsin celebrates his 68th birthday Monday, he faces a formidable leadership test. Parliament has started proceedings to impeach him, and opinion polls show his public support is close to zero.

His continuing poor health has raised questions over his fitness to govern. Yeltsin left a hospital on Saturday and went to a sanatorium to continue his recovery after two weeks of treatment for a stomach ulcer. He is expected to spend his birthday quietly with his family.

In coming days, Yeltsin must prepare his annual state of the nation speech, due in February, and he also is expected to respond publicly to a proposal by Prime Minister Yevgeny Primakov for a political truce in which the opposition-dominated parliament would drop impeachment proceedings in return for an even less-visible role for Yeltsin.

Yeltsin has given Primakov day-to-day responsibility for governing Russia and faces pressure to increase the delegated power.

"One way or other, a person usually sums up the results of the last year at his birthday table. And for Boris Yeltsin it has not been an easy one on the political or the personal level," RIA news agency said on Sunday.

RIA noted that Yeltsin retain wide constitutional powers, including control of the trigger of Russia's nuclear arsenal.

Tribuna newspaper called the situation "...a vacuum of power in Russia."

Russia remains deep in economic crisis. Its relations with the United States, Russia's most important ally, are strained.

Yeltsin's move on Saturday to the Barvikha sanatorium near Moscow highlights the impression that he has become isolated on the political scene, even if he retains the potential to exert greater influence again.

Over the last three months, he has hardly been seen in public, limiting himself to rare television speeches, and is often shown slumped in a chair at meetings at residences outside Moscow.

Although Yeltsin is unpredictable and has stunned critics before by making comebacks, he resembles a shadow of the man who dominated the political scene when he became president in 1991.

He has kept Russia on the road of democratic and economic reform. But the country has been marred in that time by repeated corruption scandals, a rise in organized crime and by a bloody, nearly two-year war in the separatist Chechnya region.

Kremlin aides say Yeltsin's long-term aim is to groom a successor for the presidential election in 2000 and ensure the vote is free and fair, permitting a smooth transition of power.

Primakov is a potential candidate to be his chosen heir in an election race likely to include Moscow mayor Yuri Luzhkov, Communist Gennady Zyuganov, regional governor Alexander Lebed and liberal economist Grigory Yavlinsky.

Despite his political and health problems, most commentators say Yeltsin's resignation is unlikely.

"One thing is clear, the resignation of the president before his term ends can be discussed only from the medical point of view," said Vyacheslav Nikonov, a leading political analyst.

"No political considerations or enemies' intrigues will make Yeltsin step down if he himself does not think he wants to."

Reuters contributed to this report.