WI Mitterrand won in '65? It appears that he swept the urban centres, while the General won the rural areas and beat him 55-45 in Round Two. You'd think de Gaulle would get a better margin...
Well, as elections go, that one wasn't close: de Gaulle won with 55% to Mitterrand's 45%, and the latter did have a very good campaign. It was already surprising that he managed to make it into the run-off. So off the top of my head I don't know what could result in his getting an even higher score; maybe if de Gaulle actually declined to campaign at all, considering it beneath him to court voters, and taking his victory for granted (after all, he won his 1962 referendum on the direct election of the president of the Republic with a solid majority).
In 1965, Mitterrand was already the single candidate for the entire left wing, from the center-left Radicals to the Communists; he hadn't formally agreed to give ministry portfolios to Communists as he would in 1981 in OTL, but he probably would have had to considering their electoral strength.