WASHINGTON — As congressional Democrats gear up for another bruising legislative push to expand voting rights, much of their attention has quietly focused on a small yet crucial voting bloc with the power to scuttle their plans: the nine Supreme Court justices.Democrats face dim prospects for passing voting legislation through a narrowly divided Congress, where an issue that once drew compromise has become an increasingly partisan flashpoint. But as they look to reinstate key parts of the Voting Rights Act of 1965, a …