The FMLN appears to have been strengthened by the elections
The party of El Salvador's former left-wing guerrillas is poised for victory in the parliamentary election.
With almost 70% of votes counted, the Farabundo Marti National Liberation Front (FMLN) had a nine-point lead over the governing conservative party Arena.
But Arena took the lead in municipal polls and neither party has gained enough seats for an outright majority.
The FMLN's victory would be their first since the end of the country's bloody civil war in 1992.
The results of Sunday's poll are seen as an indicator of how well both parties will fare in the next presidential election.
The BBC's Greg Morsbach says that although the FMLN fell short of securing the overall majority in the national assembly, it still emerged as the strongest political force in the national assembly.
The party also managed to increase its number of mayors.
However, in the municipal elections, Arena appeared to have regained control of the powerful local government in the capital, San Salvador.
Despite the mixed results, the leftist politicians are now confident they can win the presidential polls scheduled for 15 March, says our correspondent.
The party's leader, charismatic former CNN journalist Mauricio Funes, appears to have strengthened his party and persuaded many voters that FMLN is a centre-left party willing to work with Washington.
He has dismissed opponents' claims that he would lead the country into close alignment with socialist Venezuelan leader Hugo Chavez.