Politico, as usual, breaks it down:

Senate Minority Leader Mitch McConnell said he would join a group of Senate Republicans threatening a filibuster to oppose a cloture vote if Senate Majority Leader Harry Reid moves a gun bill to the floor this week.

Reid has promised to move a gun bill to the Senate floor this week, but it’s unclear which one. It depends on whether Democrats can strike a deal on background checks on gun sales — otherwise, they may settle for a less strict bill that includes some form of background checks, increase penalties for gun trafficking and increase school security. In order to get around the filibuster, Reid has indicated that he would file a cloture motion, which would require a 60-vote margin to move the bill.

“While nobody knows yet what Senator Reid’s plan is for the gun bill, if he chooses to file cloture on the motion to proceed to the Reid Bill (S. 649), Senator McConnell will oppose cloture on proceeding to that bill,” McConnell spokesman John Ashbrook said in a statement.

As Roll Call noted in an excellent article yesterday morning, Senate Majority Leader Harry Reid (D-NV) may try to invoke the truce on filibustering motions to proceed by promising both parties at least two amendment votes, though this might betray a lack of confidence about the bill’s viability. If the Tea Party senators, never mind McConnell (R-KY), filibustered the motion to proceed anyway, they would set off a political firestorm. A note to Hill staffers: Make sure you have plenty of food and water.