Investors want to know what Putin's final role will be after he steps down because they see political stability as key to Russia's booming $1.3 trillion (661 billion pound) economy.
RIDDLE NOT SOLVED
Kremlin-watchers believe the riddle of what Putin will do next is still not fully solved because the post of prime minister is an awkward one for someone so powerful.
The prime minister is junior to the president, can be sacked at the president's whim and often carries the can for policy failures. Putin filled the post with a series of low-level technocrats all seen as expendable.
Some analysts see United Russia leadership as a way for Putin to preserve long-term influence by moulding the party, closely tied to the Kremlin since its creation, into a powerful political force in its own right.
The president can sack the prime minister but he has to seek the approval of parliament - controlled by United Russia - to appoint a new premier. The party has the two thirds majority required to amend the constitution.
Others say the party's influence and unity could dissolve if the new Kremlin administration withdraws its support.
Putin helped found the party, which was designed in the last days of former President Boris Yeltsin's rule to ensure the Kremlin's control of parliament.
Russia's Vedomosti newspaper reported on Friday that Putin could take a leadership role in the party without being a member.
The paper cited unidentified Kremlin and party officials as saying that United Russia was preparing to create the post of "non-member party leader" who would define strategy rather than take part in day-to-day management.
There is a precedent for leadership of a party, rather than any state position, providing the lever of power in Russia. For much of the 20th century, the leader of the Soviet Communist party held sway over all state institutions.
