Before the Balkan Wars Macedonist ideas were shared by a limited circles of intellectuals. They grew in significance during the interbellum, both in Vardar Macedonia and among the left-leaning diaspora in Bulgaria, and were endorsed by the Comintern. During the Second World War, these ideas were supported by the Communist Partisans, who founded Yugoslav Macedonian Republic in 1944.
All contemporary sources (that I could find) lead to the name Georgi Dimitrov (head of the Commintern) and his negotiation with Josip Broz Tito on the creation of a Federation of the Southern Slavs, which had been underway since November 1944 between the Bulgarian and Yugoslav Communist leaderships. The idea eventually resulted in the 1947 Bled accord:
The preliminary plan for the federation included the incorporation of the Blagoevgrad Region ("Pirin Macedonia") into the Socialist Republic of Macedonia and the return of the Western Outlands from Serbia to Bulgaria. In anticipation of this, Bulgaria accepted teachers from Yugoslavia who started to teach the newly codified Macedonian language in the schools in Pirin Macedonia and issued the order that the Bulgarians of the Blagoevgrad Region should claim а Macedonian identity.
Are there documents that testify the existence of Macedonia and the Macedonian people, prior to the 19th century?