WASHINGTON  Even before a landmark Supreme Court ruling on campaign finance law expected within days, a series of other court decisions is reshaping the political battlefield by freeing corporations, unions and other interest groups from many of the restrictions on their advertising about issues and candidates.

Legal experts and political operatives say the cases roll back campaign spending rules to the years before Watergate. The end of decades-old restrictions could unleash a torrent of negative advertisements, help cash-poor Republicans in a pivotal year and push President Obama to bring in more money for his party.

If the Supreme Court, as widely expected, rules against core elements of the existing limits, Democrats say they will try to enact new laws to reinstate the restrictions in time for the midterm elections in November. And advocates of stricter campaign finance laws say they hope the developments will prod the president to fulfill a campaign promise to update the presidential campaign financing system, even though it would diminish his edge as incumbent.

Many legal experts say they expect the court to use its imminent ruling, in the case of Citizens United v. Federal Election Commission, to eliminate the remaining restrictions on advertisements for or against candidates paid for by corporations, unions and advocacy organizations. (The case centers on whether spending restrictions apply to a conservative group’s documentary, “Hillary: The Movie.”)