The best way to understand the current political calculation of Republican Senators on comprehensive immigration reform is to study what happened to House Democrats on climate legislation back in 2009.

AP photo.

Yes, you read that right. House Democrats '09 = Senate Republicans '13. Here's why.

In the summer of 2009, the Democratic House majority passed a climate bill that included a "cap and trade" provision on greenhouse gas emissions. The measure, which passed the House 219-212, was an extremely tough vote for many vulnerable Democratic members who came under withering attack in the moment from Republicans who said the bill amounted to a massive energy tax increase.

Then the bill died in the Senate. Less than a month after it passed the House, Senate Majority Leader Harry Reid (Nev.) conceded that the votes simply weren't there for comprehensive climate change legislation.

House Democrats who had voted for the bill were left with the worst of both political worlds. Not only had they cast a very tough vote but they also had absolutely nothing to show for it. Republicans knew a hanging political curve ball when they saw one and used that cap and trade vote to bash targeted Democrats in the 2010 election. Dozens lost, defeats that contributed in no small part to Democrats losing their House majority.

Fast forward to the current debate on comprehensive immigration reform. And put yourself in the shoes of, say, Senate Minority Leader Mitch McConnell (Ky.).

McConnell, who voted against ending debate on a border security amendment Monday that was seen as a strength test for final passage on the immigration bill, has shown a penchant for deal-making with Democrats -- most recently as the chief GOP negotiator in the fiscal cliff deal.

But, McConnell is also up for reelection in 2014. And his past deal-making has rubbed some conservatives in Kentucky the wrong way. Voting for the immigration package  would almost certainly re-irritate those forces, a fact McConnell, who may well be the savviest pol in the Senate, knows all too well.

To do so in support of a piece of legislation that had a real chance at becoming law might be a risk McConnell would be willing to take, understanding that his party badly needs to get the issue behind them, politically speaking, and that the "Gang of 8" bill represents the best chance to do that.

But, McConnell -- as well as many of his politically pragmatic GOP Senate colleagues -- see little to no chance that the Republican-controlled House will even take up the bill, much less pass it. (If there was any doubt about the inability of House GOP leaders to bend their members to their will that went out the window last week when the farm bill failed.)

Rather than walk a political plank -- as House Democrats did back in 2009 -- and run the risk of having it sawed off behind you, Senate Republicans, including McConnell, seem likely to play it by safe by voting "no" on final passage.

The lesson: Moral victories are all well and good but politicians tend to be far more concerned with actual victories.