By Andy Sullivan WASHINGTON (Reuters) – Since the U.S. Supreme Court’s Roe v. Wade decision legalized abortion nationwide in 1973, the issue has become one of the defining fault lines in U.S. politics, with Democratic politicians firmly supporting abortion rights and Republican lawmakers lining up in opposition. In 1973 the lines were more blurred. Republican and Democratic voters were equally likely to say abortion should be legal, while it was easy to find Republican officials who supported abortion rights and Democrats who opposed the procedure. So what changed? NOT A PARTISAN ISSUE AT FIRS…