The Senate will begin voting Wednesday afternoon on nine proposed amendments to President Obama's gun control bill -- all of which will need at least 60 votes to be added to the overall legislation. That high bar seems certain to doom a high profile and bipartisan effort to expand background checks with even the architects of the amendment acknowledging Wednesday morning that the votes simply weren’t there.

But, why does the background check amendment -- and the other eight amendments that will be offered -- need 60 votes rather than a simple majority? The answer is a combination of Senate procedure and the complex politics of guns.

Let’s start with Senate procedure. There are a couple of ways senators can introduce amendments to existing bills. One is to introduce them one at a time.  An amendment is brought to the floor and debate begins.

But, to end debate, on even one amendment, requires 60 votes -- a process known as invoking cloture. Rallying 60 Senators to any cause is very difficult in such a polarized body. And even if cloture is invoked on an amendment, there can be up to 30 hours of debate on it. That’s a time consuming process for one amendment – let alone nine, which is what the Senate is considering beginning today.

Given that, another route is to require a 60-vote threshold on the final vote for each individual amendment rather than to end debate on each measure. Instead of drawing the process out over days or weeks, the votes then can be taken in quick succession.

The latter scenario is the path Senate Majority Leader Harry Reid (Nev.) chose to take. He asked for the Senate's unanimous consent to require 60 votes for each of the nine amendments. Since no senators objected, that's where the bar is set.

So, why didn't Reid try to get the unanimous consent agreement to set all amendment votes at a 51-vote threshold?

Because to do that would have opened the bill up to the very likely possibility that amendments favored by gun rights advocates would be added to it.  One, for example, would allow gun owners who receive a state-issued permit to carry a concealed weapon to take that weapon into other states that issue such permits.

By agreeing to a 60-vote threshold on all the amendments, gun control advocates have made passage more difficult for the amendments that, in their view, would weaken or, more likely, kill the larger bill.

What Reid was trying to do then was to thread a very fine needle by a) keeping open the possibility that expanding background checks might make it into the bill while b) keeping amendments that would have doomed the bill out of it.

In the end, it was too difficult a task.