"I had forgotten that I had returned to France for the moment"

An overly enthusiastic apparatchik of the glorious French lay State at its consulate in Atlanta, supervising the polling station there, warned a priest, dressed in his cassock, that he would not let him vote in the second round, next month, if he returned dressed in the same way, reports Le Salon Beige.

"After putting my ballot into the urne, the supervisor of the polling
place asked me if I was a priest, to which I replied in the
affirmative.  After four years in the United States, I had become
rather naive, because I waited for him to ask for a blessing or for the
location of my parish, as so often happens here.  I had forgotten that
I had returned to France for the moment."
 
The clerical habit, the fellow said, was not itself against the law, but was an "injury to the dignity of the polling place" and "might offend others present", for which causes, evidently, the supervisor can prevent offenders from voting. 

The priest asked to see the consul who told him that of course he could vote wearing his soutane.

The priest wrote of his experience to Forum Catholique.  From comments there, it seems pretty clear that it is quite legal to wear the cassock to go to vote.