Serbia's Socialist Party -- once headed by the late strongman Slobodan Milosevic -- has agreed to support the pro-Western Boris Tadic in the country's May 20 runoff presidential election.
Tadic's Democratic Party said in a statement that the Democrats and Socialists -- which finished second and third in the May 6 parliamentary elections -- had agreed to revive their coalition.
The move could block the rightist Serbian Progressive Party from entering goverment.
Tadic, who resigned as president in April to force early polls, won 27 percent in the first round.
He will face Progressive Party head Tomislave Nikolic, who won 25.5 percent in the runoff.
The Democratic and Socialist parties have 113 seats in the 250-seat legislature and would need support from the United Regions party or the Liberal Democrats to form an administration.
Based on reporting by Reuters and AP