Milanovic and Crvenkovski held a joint news conference in Sarajevo with Bosnian SDP chief Zlatko Lagumdzija and SDP official Zeljko Komsic, who serves as the Croat member of the tripartite Bosnian presidency.
Milanovic said that he had arrived in Sarajevo guided by "his mind and soul, and confident that he was doing the right thing".
"This election is a struggle for the institutions of Bosnia and Herzegovina. Without them, there can be no state," Milanovic said, adding that it was only natural that the Croatian SDP should support Kosmic's re-election as the Croat representative in the Bosnian presidency.
Crvenkovski said he had arrived in Sarajevo to back the idea of social democracy.
Apart from Milanovic and Crvenkovski, the chief of Montenegro's SDP, Ranko Krivokapic, also attended the final rally of the Bosnian SDP in Sarajevo on Friday evening.
On Thursday, Croatian Social Liberal Party (HSLS) president Darinko Kosor visited Bosnia to support local liberals ahead of Sunday's vote.
The Croatian Democratic Union (HDZ) of Croatia has supported the HDZ BiH candidate for the Bosnian presidency, Borjana Kristo.