The 59 or so senators growing weary of the GOP's current penchant for filibustering are increasingly looking to an option that would allow them to revamp the chamber's filibuster rules with only 51 votes—which Democrats have—rather than 67, which they most certainly don't. The "constitutional option" would invoke a section of, well, the Constitution, which allows the Senate to determine its own rules—the filibuster, by preventing the Senate from voting, is then technically unconstitutional.
The option is widely considered viable, but most experts agree that it can only be used at the beginning of a new Congress. While it's never been used, it has "been repeatedly, and effectively, almost used," Ezra Klein writes in the Washington Post. And by convincing Republicans they're prepared to use it, Democrats may be able to get the GOP to compromise on the filibuster.