Tel Aviv — Israel's opposition Labor Party rejected Prime Minister Menachem Begin's offer to form a government of national unity. Labor leader Shimon Peres said after a meeting of the party's Central Committee that Mr. Begin's proposal had been overwhelmingly defeated. Labor Party spokesman suggested Begin made the proposal because of difficulties with his coalition partners. Since its formation seven months ago, the government has narrowly survived several votes of confidence in the Knesset (parliament).