Russian President Dmitry Medvedev says he decided not to run for a second term because Vladimir Putin is both more popular and more authoritative.
It did not mean next year's election result was predetermined, he said in a first interview since Prime Minister Putin revealed their plan to swap jobs.
Mr Putin's bid to return to the top post he held between 2000 and 2008 has angered the country's weak opposition.
But Mr Medvedev told Russian TV new faces would "renew" the government.
"It will be a pivotal renewal of the government - a government consisting of new people. This is fundamentally important," he said.
Mr Medvedev had previously criticised "stagnation" in the Russian political arena, which is heavily dominated by the pro-Kremlin United Russia party, saying it is damaging to both ruling and opposition forces.
But United Russia has approved Mr Putin's proposal that Mr Medvedev heads the party list for December's parliamentary elections and become prime minister after the presidential poll.
In the interview, to be broadcast later, Mr Medvedev said the decision to swap roles had been taken years ago but that he had kept open the option of running for president, depending on his popularity.
However, he said: "Prime Minister Putin is now unquestionably the most authoritative politician in our country and... his rating is somewhat higher."
The BBC's Steve Rosenberg in Moscow says Mr Medvedev is keen to show he is not a Putin puppet, who is simply been keeping the presidential seat warm until Vladimir Putin returns to the Kremlin.
That is how many Russians now see him, and this interview will have done little to change this image as the Kremlin caretaker, our correspondent says.
Analysts say there is little doubt that Mr Putin, 58, will win the presidential poll in March.
The country's small liberal opposition has greeted the prospect with dismay, while Finance Minister Alexei Kudrin - who had hoped to take up the prime ministerial role - resigned after a public row with Mr Medvedev.
Critics have included former Soviet leader Mikhail Gorbachev, who warned that Russia risked "wasting six years" with Mr Putin in charge.
Meanwhile Boris Nemtsov, a deputy prime minister under Boris Yeltsin and co-founder of the unregistered People's Freedom Party, predicted "giant corruption" in the country's politics.
But Mr Medvedev insisted the decision over who ran the country would ultimately rest with the Russian people.
"How can [the elections] be predetermined? Such talk is completely irresponsible, misleading and even provocative," he said.
"The choice is made by the people, and these are not empty words - that's absolutely the way it is."
Mr Putin, who has already served two terms as president, was barred by the constitution for running for a third consecutive term in 2008.