WASHINGTON — As long as they are rivals, Elizabeth Warren and Bernie Sanders couldn’t stay friends forever. The White House hopefuls and longtime allies have spent the past year throwing red meat to the party’s progressive base with calls for massive structural reforms to the nation’s political and economic life. The differences that existed — how quickly to transition to government-funded health care, how broadly to apply a wealth tax — were generally around the margins. But as their he said-she said feud over whether Sanders told Warren a woman could not be elected president shows, the friendship ends as the voting begins.