HARARE, Zimbabwe — Zimbabweans mobilized Monday for what they hope is a major push to oust President Robert Mugabe, an increasingly isolated figure who faces impeachment proceedings and more street demonstrations as he ignores calls to resign. While there is a national consensus that the 93-year-old president should go after nearly four decades in power, Mugabe has refused to step down as leader of a country that has known no other head of state since independence from white minority rule in 1980.