The Republic on the Move party made clear that even top-rank politicians from established parties, like Valls, were not guaranteed a slot on its list of parliamentary contenders. His right-wing rival Marine Le Pen secured only 33.9 percent of the votes.
So far the names of only 14 candidates are known, but on Thursday afternoon "around 450" will be revealed, Jean-Paul Delevoye of Republic on the Move said.
To qualify, would-be candidates, including total newcomers to politics, filled out extensive online applications with CVs and explanatory letters for pre-screening and follow-up interviews, according to local media interviews with some of them.
Catherine Fabre, a middle-aged business management lecturer at Bordeaux University, told BFM TV she was in the running and wanted to shake things up in a National Assembly which was, in her word, dominated by too many middle-aged white men.
Emmanuel Macron's victory shows France's commitment to the European Union and Refugees.
But he now needs to consolidate, knowing that many of those who voted for him were driven less by enthusiasm for him than by a desire to stop Le Pen.
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Baroin tried to highlight the risks facing defectors by citing the example of Manuel Valls, a former Socialist prime minister who said he was ready to back Macron in the June election.
Macron's new party is casting its net wide as it seeks support for a majority that will allow him to push through reforms to revive an economy dogged by high unemployment and poor growth.
A poll showed that only 52 percent of voters want a pro-Macron government to emerge from the elections, while 42 percent favoured a legislature that would be a check on the new leader.
President Francois Hollande warmly greeted President-elect Macron in their first public meeting since the victory.
Several senior members of his Socialist Party have already announced plans to launch new movements, inspired by divisions in the party ranks and perhaps by Macron's success.
Macron, who will be inaugurated on Sunday, has said he is aiming for an absolute majority in the lower chamber in June's elections.