The failure of an amendment to expand background checks on gun purchases -- the signature piece of a legislative package backed by the White House to curb gun violence -- ends a journey that began in late December when 20 children and six adults were murdered in Newtown, Connecticut.

A gun.

Studying the path from Newtown to the vote on the amendment offered by Sens. Joe Manchin (D-W. Va.) and Pat Toomey (R-Pa.) that came up short of the 60 votes it needed is an instructive exercise in how Washington works.

We've spent a lot of time thinking about what the failure of the background checks compromise means. Our lessons learned are below.

* Newtown didn't change things. For weeks after the massacre in Connecticut, there was a broad sense -- among Democrats and Republicans -- that something fundamental had changed when it came to the debate over the proper place for guns in society. Unlike Columbine, Virginia Tech and Aurora, the slaughter of 20 children meant that the government would have to act -- the American public would demand it.

But then time passed. And, as Newtown receded as a top-of-the-mind issue so, too, did the predictions that what happened in Connecticut had changed the political math on guns.  The White House's proposed gun legislation was stripped of a renewal of the assault weapons ban and strictures on the sale of high-capacity magazines, an acknowledgement from Democrats that adding either measure to the overall package would doom it.

The belief was that expanded background checks was the one major pillar of the gun legislation that might be able to make it through the gauntlet of Republican opposition and wariness from Democrats up for re-election in conservative states in 2014. That belief proved to be wrong.

* The 2014 map mattered more than the national polls. Expect the White House to turn blame on Republicans in the wake of the failure of Manchin-Toomey. The truth, however, is that this amendment was doomed as much by an unwillingness of Democrats up for re-election in 2014 to get on board with it as by Republican opposition to it.

Democratic Sens. Max Baucus (Mont.), Mark Pryor (Ark.) and Mark Begich (Alaska) simply did not want to vote for this background check amendment and, by holding off their support, made the math absolutely unworkable once the likes of Republican Sens. Dean Heller (Nev.), Kelly Ayotte (N.H.) and Richard Burr (N.C.) came out against it.

And, why did they not want to vote for it? Because of the belief that doing so would hurt their chances of winning re-election next November in states that President Obama lost by significant margins and that contain large swaths of rural areas where gun rights are considered sacred.

If you are a gun control advocate looking to assign blame, Senate Democrats (or at least some of them) deserve as much of it -- if not more -- than their Republican colleagues.

* Passion is the sine qua non of politics. We -- and everyone else -- has been asking one question over and over again these past few weeks: How is something, like expanded background checks, that is supported by between 80 and 90 percent of the American public not a no-brainer for Congress to pass?

The answer is passion. That is, the number of people who prefer more gun control rather than less don't -- by and large -- feel that way with a deep burning desire in their soul. Those who support gun rights and see any attempt to limit them as a slippery slope toward confiscation, on the other hand, feel incredibly passionate about the issue. The gun-rights supporters are a smaller but far more vocal and active group than those who want more restrictions on guns.

All of the above means that politicians believe -- whether they are right or not is up for debate -- that they face more negative consequences for voting against gun-rights legislation than voting against gun-control measures.

The fate of the overall gun package remains to be seen. And, there is a possibility -- albeit a slim one -- that the Senate could re-consider the Manchin-Toomey amendment at some point in the future.

But, it's been clear for weeks that the White House and gun-control advocates had put all of their hopes on the expanded background checks provision. If that doesn't make the final bill, the effort will be regarded as a failure by many who believed that new gun-control legislation's time had come.