Unaccustomed as political scientists are to florid language, they have nevertheless come up with the ‘theory of the dyke’, to explain the continuing success of the nationalist and identitarian Rassemblement National. A dyke can hold back the flood for so long, but once water has overflowed there is no getting it back. When in 2002 Marine Le Pen’s father, against all odds, beat the socialists to go through to the presidential run-off against Jacques Chirac, there was no reversing the flow.

Marine Le Pen’s score of 23.1 per cent in the first round of the election this week is the highest in the nationalist right’s 50-year history. Now we are faced with the closest run-off since 1974 when Valéry Giscard d’Estaing beat Francois Mitterrand 51 to 49. Whoever wins on 24 April this election is breaking many baleful records that lay bare a deeply fragile and unsettled political system.

Sunday saw the highest abstention rate since 2002 and one of the highest of the Fifth Republic. For the first time in the Fifth Republic’s history over half the votes went to anti-system candidates. This is all the more dangerous for these candidates historically having minimal representation in the 577-seat parliament. For all her 8 million votes Marine Le Pen’s party is represented by seven MPs. Jean-Luc Mélenchon, who came third (on 22 per cent) with 7.6 million votes, is represented by 17 MPs. Another four and a half million votes, including Eric Zemmour’s (7 per cent) 2.4 million, have no parliamentary representation at all. By contrast Anne Hidalgo’s (1.7 per cent) Socialist group has 31 MPs. The system is broken. In 2017 Emmanuel Macron pledged to eliminate the extremes from French politics with a dose of proportional representation, but nothing came of it. On Sunday night street riots ignited in the Lyon suburbs and Rennes, where 500 anti-capitalist, anti-fascist, anti-globalist youth smashed street furniture and bank frontages.

The first round was characterised by more negative voting than previously. Votes were cast not so much vote for manifestos than to eliminate candidates: to keep out Macron or Le Pen. The Gaullist and Socialist parties that have structured politics for over 60 years have imploded and account for less than 7 per cent of the electorate. Neither they nor the Greens will have their multi-million pound campaign expenses reimbursed, adding to their plight.

Of course Macron consciously began the process of traditional party dissolution when in 2016 his En Marche party burst on to the political scene around his manifesto book Révolution. The genie is now out of the bottle.

Party realignment on a scale not seen under the Fifth Republic will be the order of the day in the 12 and 19 June legislative elections, whoever wins the second round. This will decide whether the new president has a governing majority – by no means a firm prospect. France’s political landscape will be reconfigured into the three blocs that reflect today’s top three candidates: a new radical left around Mélenchon’s France Unbowed party, a new radical right around Le Pen’s Rassemblement National and a new grouping that Macron quietly announced on Sunday to gather the remnants of centre left and right.

All of this is a reflection of the evaporation of traditional party loyalty. Still more interesting is the porosity between radical right and radical left, not seen since the 1930s. Last night Jean-Luc Mélenchon coaxed his voters not to cast a ballot for Marine Le Pen in the second round, while refusing to recommend voting for Macron. But opinion surveys show that a third will ignore that diktat, as will many who voted for ecologist, socialist, communist and Trotskyist candidates. The appeal for Mélenchon voters to switch to Marine Le Pen in the second round, and in the June legislative elections, is motivated by the Rassemblement national’s similar demographic and the clearly socially grounded programmes of both.

Whoever wins, the upended parliament resulting from the legislatives may be the Fifth Republic’s most ungovernable, with its three clashing blocs. Under the current parliament Macron has a working majority. But the binary left-right Fifth Republican politics baked into the regime by General de Gaulle in 1958 could have disappeared from June. The regime that it was designed to replace, that of the Fourth Republic, was characterised by three hostile blocs, Communists, Gaullists and the centrist ‘Third Force’ (Christian Democrats and socialists). It was also characterised by endemic ministerial instability and immobilism.

How would a Macronist prime minister get his legislation past a blocking radical right and radical left as happened under the Fourth Republic? How would Macron push back the pension age from 62 to 65 or continue his contested mission of labour reform and benefits? Could the final record of this election be to condemn the present regime to the fate of what de Gaulle – referring to the Fourth Republic – called ‘the disastrous regime of parties’?