BELGRADE, Serbia — Powerful Serbian Prime Minister Aleksandar Vucic was on his way to a landslide victory Sunday in a presidential election that was a test of his authoritarian rule amid growing Russian influence in the Balkan region. A projection by independent pollster Ipsos Strategic Marketing had Vucic receiving more than 55 percent of the votes cast — more than enough to claim the presidency outright. The polling agency’s projection showed liberal challenger Sasa Jankovic placing second with 15 percent and Luka Maksimovic, a media student who ran as a parody politician, coming in third with 9 percent. Vucic, a former ultranationalist who is now a declared pro-European Union politician, was already slated to win the presidency by a high margin against 10 opposition candidates. Jankovic, an independent candidate with no party affiliation, said Sunday he was happy with his campaign, which has galvanized the pro-democratic movement in Serbia that has been upset with the country’s persistent corruption and growing autocracy.