A politician campaigning to end Russia’s war in Ukraine has been barred from running against Vladimir Putin in March presidential elections.

Russia’s Central Election Commission ruled Thursday that Boris Nadezhdin, a member of the Civic Initiative party, hadn’t met the conditions for registration as a candidate, the state-run Tass news service reported. Officials said almost 10,000 of some 105,000 signatures from Russians in support of Nadezhdin weren’t verified.

Nadezhdin said in a post on Telegram that he’d appeal to Russia’s Supreme Court to overturn the ban, saying he’d collected more than 200,000 signatures from all over the country from Russians who often braved freezing temperatures to add their names to the petitions.

“We conducted the collection openly and honestly,” he said. “I’m not backing down.”

Nadezhdin, 60, said in a campaign manifesto that he’d entered the election race “as a principled opponent of the current president’s policies,” saying that Putin had “made a fatal mistake” in starting the February 2022 invasion of Ukraine.

Already the longest-serving Kremlin leader since Josef Stalin, Putin is seeking a new six-year term in the March 17 elections as Russia’s war in Ukraine approaches its third year. The result is widely expected to be a formality.