Can the government ban campaign ads 60 days before an election?

There is a big campaign finance case to be heard by the US Supreme Court today.

The disputed regulation barring corporate and labor TV and radio ads as much as 60 days before an election is part of the Bipartisan Campaign Reform Act, widely known for its Senate sponsors, Mr. McCain and Russ Feingold (D., Wis.).

The provision aims to keep corporations and unions from using their wealth to unfairly influence elections, says Scott Nelson, an attorney with advocacy group Public Citizen. If the FEC loses, Mr. Nelson says, corporations and, to a lesser degree, unions, could spend "hundreds of millions of dollars" in the run-up to the next election for negative ads. . . .

My own research has shown that these various regulations protect incumbents from competition. But I have a hard time believing that interest groups taking out ads within 60 days before an election is not protected political speech. If individuals can't speak out on issues before an election what else is left to free speech?