The last 18 hours haven't been good ones for advocates of expanding background checks for gun purchases.

Sen. Pat Toomey and Sen. Joe Manchin. Getty photo.

The decisions by Republican Sens. Jeff Flake (Ariz.) and Richard Burr (N.C.) to oppose the deal cut by Sens. Joe Manchin (D-W.Va.) and Pat Toomey (R-Pa.) on background checks makes the path to 60 votes for the amendment very, very difficult.

Let's start with the math. (And make sure to bookmark this post where we are updating the vote count on Toomey-Manchin as Senators make their intentions public.)

There are 52 "yes" votes and 39 "no" votes. There are eight Senators regarded as "undecided" and another -- New Jersey's Frank Lautenberg -- who may or may not be able to make it to the vote due to health issues.

To get to 60 votes -- the threshold needed to add the amendment to the overall bill -- Manchin and Toomey need a) 7 of the 8 fence sitters AND Lautenberg or b) all 8 undecideds without Lautenberg.

Running the table or claiming seven of their eight votes seems very unlikely at the moment -- for several reasons.

The biggest factor is the makeup of the undecided eight.  Three are Republicans (Kelly Ayotte of New Hampshire, Dean Heller of Nevada and John McCain of Arizona) while five are Democrats (Max Baucus of Montana, Mark Begich of Alaska, Heidi Heitkamp of North Dakota, Mary Landrieu of Louisiana and Mark Pryor of Arkansas).

All three Republicans voted for cloture last week, making them at least open -- in theory -- to voting for the Toomey-Manchin amendment.  But, of the 16 Republicans (including Ayotte, Heller and McCain) who voted for cloture, 10 have since announced they will vote against the amendment while three (Toomey, Mark Kirk of Illinois and Susan Collins) have said they will vote for it.  Given those numbers -- plus the recent announcements by Flake and Burr -- it's far more likely that the undecided GOP trio come out against Toomey-Manchin than for it.

But, for the sake of argument let's say that the three undecided Republicans decide to be for the bill. That would mean that either four (if Lautenberg votes) or all five Democrats (if he doesn't) who haven't publicly stated where they stand would need to be for the bill for it to pass.

That seems very unlikely when you remember that both Begich and Pryor voted against cloture last week -- the only two Democrats to do so. Both men are up for re-election in states that not only went strongly against President Obama in the 2012 election but also are heavily rural and generally supportive of gun rights.

And, none of the other three Democrats are easy "yes" votes either. Baucus and Landrieu voted for cloture but are up in conservative-tilting states in 2014. Heitkamp was just elected in 2012 but is keenly aware of having to keep her distance from the national party if she wants to stand a chance at getting re-elected.

If Democrats lose both Pryor and Begich, the amendment fails. If they lose one of the two and any one of Baucus, Landrieu and Heitkamp, the amendment fails. And, remember, that math is based on the somewhat questionable assumption that the three remaining Republican undecideds decide to vote for the Toomey-Manchin amendment AND that Lautenberg returns to vote for it as well.

Toomey and Manchin -- as well as the White House -- are keenly aware of that math. And that's why they are trying to adjust the bill to broaden its appeal. (The duo are "discussing the possibility of adding language that would exempt select far-flung communities in Alaska and North Dakota from some background check requirements," report the Post's Ed O'Keefe and Tom Hamburger.)

Any time you take a carefully negotiated compromise bill and try to tweak it to get a vote or two, you run the risk of losing someone (or someones) with the changes.  These bipartisan bills are negotiated to within an inch of their legislative life before they ever see the light of day so the assumption that there is plenty of wiggle room to bring on additional votes may not be right.

And, think of the politics of this bill for those who are announced "no" votes. Political reality dictates that going from "no" to "yes" is a very tough proposition.

Yes, if the Toomey-Manchin amendment is changed in some fundamental way, any Senator who goes from "no" to "yes" can cite the changes when pressed on the reason for his/her change of vote. But, that assumes that his/her constituents are deeply engaged with the intricacies of the legislation (they aren't) and that a complex argument can beat a simple one in the context of a political campaign (it can't). (The complex argument explains why the switch made sense; the simple argument is that the switcher is a reed blowing in the political wind.)

Being labeled a flip-flopper is never a good thing. Being labeled a flip flopper on guns -- particularly in a conservative-tilting state -- is a very bad thing.

Taken all together, it's hard to see the math for the Toomey-Manchin amendment adding up. (As we always note: Politics is a changeable business. So, what is true today may not be true tomorrow. It's always possible that Toomey and Manchin will find a way to save the bill.)

If the background checks amendment fails, that would mean that the three pillars of the attempt to change gun laws -- assault weapons ban, ban on high capacity magazines and background checks -- would all fail to make the final bill. And that would be a stunning result given that we are less than four months removed from the tragedy in Newtown.