Emmanuel Macron has made it clear that he will not accede to demands that he bring his maverick candidacy within the broad tent of the Socialist Party. And why would he do otherwise? The moment he stuck his nose inside the tent, the people now trying to woo him with the siren song of Unity will begin to whack away at it. Outside, he's still a novelty; inside, he's just another contender.
His only worry, as the new kid on the block, is assembling the required 500 parrainages. Apparently, he doesn't have them yet, so he's appealing to France's 35,000 mayors to help him out. Of course, most of those mayors belong to political parties that have an interest in locking him out of the race, so he may have difficulty getting them. He has had no trouble raising money: although he enjoys substantial backing from wealthy donors, he claims that most of his money comes from small donations. But getting the parrainages of élus, a peculiarity of the French system, may prove to be a greater obstacle.
Macron's decision makes good strategic sense. Inside the Belle Alliance Populaire primary, he and Valls would divide the social-liberal reformist vote, with the likely result of making Montebourg the winner. Since Valls is bogged down by all of Hollande's baggage, Montebourg may still win the primary even with Macron out, leaving a 3-way contest between him, Mélenchon, and Macron for the "left" of the political spectrum in round 1, but then Montebourg and Mélenchon would whittle away at each other's base, allowing Macron perhaps to top both and thus positioning him nicely for a 2022 presidential run, which may well be his real objective. Even if Valls is the candidate, Macron could still beat him in round 1, with the same result: demonstrating his inevitability for 2022.
Or, then again, the Macron bubble may well collapse. It's really hard to say. But at this point he seems to be playing the hand he's been dealt as well as can be expected.