WASHINGTON — Supreme Court nominee Amy Coney Barrett signed a 2006 newspaper ad sponsored by an anti-abortion group in which she said she opposed “abortion on demand” and defended “the right to life from fertilization to the end of natural life.”The ad, which had more than 1,200 names attached to it, appears to be the most direct expression of Barrett’s opposition to abortion and is sure to intensify debate that she would vote to restrict, if not overturn, abortion rights if she is confirmed to the Supreme Court.It was …