The left has agreed to present joint lists in 19 of the 22 regions of mainland France, leaving the UMP in position to take control of only one, Alsace, if voting follows the same pattern as in last Sunday's first round.
The three major left-wing groupings - Martine Aubry's Socialist Party (PS), Cécile Duflot's Europe-Ecologie and the Front de gauche made up of the Communist Party and socialist defectors - met in central Paris overnight to negotiate terms.
Their combined vote hit 47 per cent last Sunday, while the UMP and its allies mustered only 26 per cent.
On Tuesday morning they announced that they will stand together nearly everywhere. But the ecologists will stand alone in Brittany in the west, while the Front de gauche will do so in the central region of Limousin and Nord-Pas-de-Calais on the Channel coast.
The Socialists were clearly ahead in all these regions, leaving the UMP no hope of winning there.
Duflot told France Inter radio that the agreement was a compromise but that compromise is healthy for democracy.
The ecologists are reported to have convinced the PS to issue a single-priced travel card for the Paris region's public transport by 2012 and to freeze investment in the Iter nuclear power project in the south.
Duflot would not be drawn on advice to voter in Languedoc-Roussillon, where Socialist renegade Georges Frêche is well ahead.
Apart from Alsace, the right could win in the overseas territories of Guyana, in the West Indies, and Réunion, in the Indian Ocean.
To add to the UMP's problems, Jean-Marie Le Pen's far-right Front National did unexpectedly well in the first round, taking back some of the votes it lost to Sarkozy in the 2007 presidential election.