Prime Minister Junichiro Koizumi was re-elected president of Japan's ruling Liberal Democratic Party on Saturday, defeating three challengers with a promise to stand by his efforts to revive this country's sputtering economy. Koizumi won a comfortable majority of the 657 ballots - 357 cast by the LDP's members of Parliament and 300 calculated to reflect the votes of the party's 1.4 million rank-and-file members. Koizumi had 399 votes, 260 more than his closest rival. Koizumi, who has held the post for 2 1/2 years, had been strongly favored to defeat the three lawmakers running against him - Shizuka Kamei, a senior party executive; Takao Fujii, a former transportation minister; and Masahiko Komura, a former foreign minister.