PARIS—French police officers held a huge demonstration outside parliament Wednesday to press for a law that protects the protectors who are feeling vulnerable to attacks, angry, and useless. The rally by security forces represented a bold and unusual move for members of an institution that stresses duty and discretion. The protest also morphed into what looked like a campaign stop for politicians ahead of regional elections next month and a presidential race next year, with security a top concern. Interior Minister Gerald Darmanin showed up at the start, squeezing through a packed crowd of hundreds waving labor union flags. Representatives from the far-right National Rally and a multitude of other parties were expected to attend. The politicians clearly hope to send a message that security matters and police officers, a considerable voting pool, are their friends. “You must help us, Mister Minister,” an officer said with emotion to Darmanin. “Every …