ATHENS, Greece — The brief but intense campaign in Greece's critical bailout referendum ends Friday, with simultaneous rallies in Athens supporting "yes" and "no" answers to a murky question in what an opinion poll suggests could be a very close vote. Prime Minister Alexis Tsipras called the referendum last weekend, asking Greeks to decide whether to accept creditors' proposals for more austerity in exchange for more loans — even though those proposals are no longer on the table. Tsipras is advocating a "no" vote on Sunday, saying it would put him in a stronger negotiating position to seek a better deal for Greece within the eurozone.