There is nothing in European Union law which prohibits a member of the European Commission from participating in a political campaign. The Commission has in the past made clear its view that its members are political figures who exercise a political role, within the parameters of which they retain the right to express their personal opinions freely on their own responsibility.
It is entirely appropriate for Commissioners to offer their views on issues of relevance to the Union, and that it would be quite wrong to seek to prevent them from so doing.