WASHINGTON — Anti-abortion protesters began to gather Friday in the nation’s capital with spirits high and a sense that the country has reached a pivotal moment that could lead to a sweeping rollback of abortion rights in many states. The March for Life, for decades an annual protest against abortion, arrives this year as the Supreme Court has indicated it will allow states to impose tighter restrictions on abortion with a ruling in the coming months — and possibly overturn the landmark 1973 Roe v.