Why did the governor endorse a senate candidate so early?

Grace is probably correct about the math.

That was his main rationale for bestowing an early endorsement on Public Service Commissioner Foster Campbell of Bossier Parish, despite the presence of two other announced Democrats in the race.

“It’s not just theory or conjecture that we can win races in Louisiana, but we have to work together,” Edwards said. “The best shot for us to be successful is to have one candidate.”

That doesn't necessarily explain why Edwards thinks the "one candidate" ought to be Campbell, though.  Rather than get too far into the relative virtues of Campbell vs Fayard (there will be plenty of time for that later!) I'd be very curious to hear what specifically the governor sees in one that he does not see in the other.

As for the third (ostensibly) Democratic candidate in the Senate race, he seems to be working with a different sort of calculus. In contrast to the strategic math which favors one candidate rather than several, Josh Pellerin has spent much of his career in business buying as many candidates as he can.