Like any election day politician confident of victory, Leonid Brezhnev smilingly cast ballots today for the Supreme Soviet, the national parliament, and then tarried some banter with the reporters who cover him.

With the ease of a long-time successful officeholder, Brezhnev was by turns reassuring, charming and just the tiniest fractioninscrutable as he talked with reporters allowed into his polling place behind his apartment on Kutuzovsky Prospekt in central Moscow.

Asked how he feels, the Soviet president and Communist Party leader replied, "Healthy," and raised his arms to show his vitality. His wife, Viktoria, who was arm-in-arm with him, demurred slightly. "Good," she said of h is health, which is uncertain.

"We won't quarrel about it," Brezhnev said with a smile.

He was asked about the Chinese invasion, which has brought two stiff Keremlin warnings to Peking to withdraw. "Why don't you ask them?" he shot back.

He was then asked when he would be ready to sign a new strategic arms limitation agreement with President Carter.

"When the documents are prepared by [Secretary of State Cyrus] Vance and our minister [Foreign Minister Andrei Gromyko], we will go and sign them," Brezhnev replied. Then, before any other questions could be asked, he turned and, closely escorted and guarded by KGB secret police, he left the polling place.

Today's elections, with 1,500 candiates for 1,500 seats, are for the two houses of the Supreme Soviet. The largely ceremonial body meets for a few days each year to rubber stamp decisions of the leadership. Brezhnev is a member from the Bauman district of Moscow and votes in his home district.