I know this is a bit obscure topic outside Spain. Before the 1977 election in Spain (the first democratic election after Franco's death) for the Cortes that redacted and approved the Constitution, there where proponents of a united left bloc composed by PSOE, PCE, PSP and other minor forces on the left of the PCE like the ORT and the PTR. Due to strategical considerations (and the attitude of the respective leaderships) it never became a reality.
Now, if we see the results, it's clear that a united left-wing bloc would have had a decisive effect in the final results. Combined, the PSOE (29'32%) PCE (9'33%) and PSP (4'46%) got 43'11% of the vote or 8'5 million votes, against 7'8 million votes got by the forces of francoist reformism (UCD and AP) Then you have the half million votes got by the catalan nationalist right and almost 300.000 for the PNV
However, thanks to D'Hondt Law, this result didn't traslated in a left-wing parliament, rather the oppositie, the right-wing forces got a majority of the seats and thus a decisive voice in the new constituiton.
So, what would be the effect of a left-wing Cortes in the new constitution? Amongst the initial proposals of the PSOE, PCE and PSP there where points about direct democracy, coopearitve economy, federal state and right of self-determination... not to mention that the leader of the PSP was an actual expert in constitutionalism...
Any ideas? Would have been a break-up with francoist institutional framework instead a reform of it possible in this scenario?