With the ruling Socialists facing a drubbing, President Francois Hollande is expected to react by ordering major changes at the top of his beleaguered government. Popular Interior Minister Manuel Valls is widely tipped to replace current premier Jean-Marc Ayrault.
The vote, in which the first ballots were cast in the French Pacific territory of New Caledonia, is also predicted to result in the far-right National Front establishing an unprecedented footprint in France's local government by winning control of as many as a dozen mid-sized towns.
But all the signs are that it will be extremely close after a better-than-expected first-round showing by Nathalie Kosciusko-Morizet, a former minister for the centre-right UMP party who is Hidalgo's rival to join a very small club of women who have run major cities around the world.
The FN also performed better than expected in the first round last Sunday and the results generally were interpreted as a signal of exasperation amongst voters with the Hollande government's failure to get a stagnant French economy moving again and reverse the upward march of unemployment.
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The president's former partner, Segolene Royal, is also tipped for a comeback following his separation from girlfriend Valerie Trierweiler in January after revelations of his affair with actress Julie Gayet. Trierweiler reportedly vetoed Royal from being included in Hollande's first candidate despite the mother of his four children being a long-established Socialist Party heavyweight.