Wonder why you’re seeing so much corporate- and union-sponsored political advertising this election cycle? You have the anti-abortion movement to thank for it.

The group Wisconsin Right to Life paved the way last year when it won a Supreme Court challenge striking down a key provision of the 2002 McCain-Feingold campaign finance law on First Amendment grounds. Under that law, corporations and labor unions were barred from running so-called issue ads on television or radio within 60 days of a general election if the spots mentioned a candidate’s name. But on June 25, 2007, Chief Justice John Roberts, writing for a 5-4 majority, agreed with the anti-abortion group that this portion of the law represented an unconstitutional restriction on free speech. As long as the ads in question don’t explicitly urge voters how to vote, they should be allowed, he wrote.

The Supreme Court ruling came too late to give Wisconsin Right to Life what it originally sought — the right to run ads in fall 2004 mentioning the state’s two Democratic senators, Herb Kohl and Russ Feingold, and urging that citizens call them to express opposition to Senate filibusters of President Bush’s judicial nominees.

Fast forward to 2008, though, when the impact of the decision is just now being realized. Some examples:

• From the start of September through October 10, the U.S. Chamber of Commerce spent $8.8 million on issue advertising that mentions candidate names — specifically, the Senate candidates in Colorado, Kentucky, Louisiana, Maine, Minnesota, Mississippi, New Hampshire, New Mexico, North Carolina, Oregon, and a handful of House candidates.

• Meanwhile, the United Mine Workers of America reported on October 9 that the union plans to spend $200,000 on a radio advertising campaign called “Coal’s Future,” targeting John McCain. UMWA President Cecil Roberts has written that nuclear power supporter McCain “has been on the attack against the coal industry for years.” And the mine workers’ leader has campaigned aggressively for Barack Obama, especially in the battleground (and coal-producing) states of Pennsylvania, Virginia, and West Virginia.

• Also, the union-sponsored American Rights at Work, headed up by former Michigan Democratic Congressman David Bonior, which advocates federal legislation making it easier for workers to form unions, spent $2.3 million on broadcast issue ads in September. The labor group’s blitz focused on Senate candidates in Alaska, Maine, Minnesota, New Hampshire, and Oregon. (The group so far has reported no ads in October.)

Of course, none of these ads tell the viewers or listeners exactly how to vote. The Chamber’s ads instead say, for example, “[Minnesota Democrat] Al Franken backs a bill to take away your private vote in union elections,” and “[Maine Republican] Susan Collins voted to ensure Maine seniors had better access to health care and affordable prescriptions.” American Rights at Work ran spots that said, for instance, “Call [Alaska Republican] Senator Stevens and Murkowski and tell them to support the Employee Free Choice Act and stop siding with wealthy CEOs over working families.”

The ads walk the line because the Supreme Court kept in place an earlier ruling that “express advocacy” by the corporations and labor groups still could be prohibited within 60 days of a general election. Ads would still be barred if they were “susceptible of no reasonable interpretation other than as an appeal to vote for or against a specific candidate.”

The Federal Election Commission issued a new rule and 17-page explanation in December attempting to interpret what that means — no mention of the election or voting, no taking a position on a candidate’s character, qualification or fitness for office. And the ads should focus on a legislative, executive, or judicial issue. In other words, the FEC will probably have to be looking at an avalanche of radio and TV commercials on a case-by-case basis, so expect plenty of debate on exactly how far the advertising can go.

And who is testing the waters already? Anti-abortion advocates. The National Right to Life Committee on September 26 asked the FEC to issue guidance on whether it can run two hard-hitting radio spots in the coming weeks. They say “Barack Obama was responsible for killing a bill to provide care and protection for babies who are born alive after abortions.” One ad says Obama has an “extreme position” and the other concludes, “Barack Obama: a candidate whose word you can't believe in.” The FEC is expected to decide before the election whether that’s a clear appeal to vote against Obama, and whether the airwaves will be open to similar spots by corporations and unions in the weeks ahead.