MOSCOW, Dec. 29 — One of Russia’s opposition parties has challenged the Kremlin’s whirlwind legislative campaign to extend the term of the Russian presidency, saying it violates a law requiring parliament to wait a year before ratifying a constitutional amendment.

The protest by the pro-democracy Yabloko party could prove an unexpected hurdle for President Dmitry Medvedev’s plan to extend the presidential term from four years to six years — a proposal that has prompted widespread speculation that his patron, Prime Minister Vladimir Putin, is preparing to return to the nation’s top post.

The Kremlin has moved unusually quickly to enact the amendment, pushing it through both houses of parliament and all 83 of the nation’s regional legislatures in less than 50 days. The upper house of parliament confirmed the votes from the regions and sent the measure to the Kremlin for Medvedev’s signature last week.

But in a statement issued the same day, Yabloko objected to the move, pointing out that a clause in the 1998 law setting procedures for amending the constitution says the regions must be given a year to consider proposed amendments. Another clause says the upper house should confirm the votes by the regions in its first meeting after that year has passed.

“They’re completely ignoring the law,” said Sergei Mitrokhin, chairman of Yabloko. “Unfortunately, this happens quite often, but this is the first time the process has been ignored for such a significant issue as a constitutional amendment.”

Mitrokhin noted that the 1998 law does not provide an exception in the event the regions approve an amendment before a year has passed, and he argued it was written that way to prevent “legislators from making such important decisions so quickly.”

The one-year requirement is especially important for regions scheduled to hold elections in March, he said, because it would give newly elected lawmakers a chance to overturn the approval of their predecessors.

A spokesman for Sergei Mironov, chairman of the upper house, did not return phone calls or respond to a written request for comment.

Yabloko urged Medvedev to send the amendment back to the upper house and said it would seek to repeal the amendment if he did not. But Mitrokhin acknowledged that his party’s objections would probably be ignored.

The issue has received limited coverage in the Kremlin-controlled news media, and the party’s lawyers have concluded that there is no way to challenge the amendment in court if it is enacted. Yabloko won less than 2 percent of the vote in last year’s parliamentary election, which critics say was unfairly tilted toward the Kremlin’s allies.

The amendment would be the first substantive change to Russia’s post-Soviet constitution since it was adopted in 1993. The Kremlin’s critics have attacked the measure as a hurried attempt to shield Putin from mounting public discontent over the country’s worst economic crisis in a decade.

Putin served two terms as president and was barred by the constitution from seeking a third consecutive term this year. He continues to wield tremendous power as prime minister, but analysts say he would be better able to weather the crisis if Medvedev resigned and he returned to the presidency with a six-year term.

The move to amend the constitution has coincided with two other Kremlin legislative proposals that the opposition says may be part of a strategy to clamp down on unrest caused by the economic crisis.

One measure, passed by parliament this month and awaiting Medvedev’s signature, would strip defendants charged with crimes related to terrorism or treason of the right to jury trials. They would instead face judges.

Critics, including prominent members of the legal community, have said the proposal would open the door to abuses by Russia’s already influential security services. But the Kremlin says the change is necessary because juries have acquitted several suspects despite incriminating evidence.

A second Kremlin proposal would expand the legal definition of treason and espionage. Human rights groups say the bill is so broadly worded it could mean a return to Soviet-style prosecutions of government critics as traitors, making crimes even of conversations with foreign reporters and nongovernmental organizations.

But Kremlin officials have not spoken publicly about the measure, and it is unclear if it will be adopted in its current form. A committee of the Duma, the lower house of parliament, is expected to begin deliberations on the proposal next year.

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