A new Washington Post-ABC News poll of the Republican presidential race shows support for Texas Gov. Rick Perry crashing and support for businessman Herman Cain increasing almost as dramatically. They are now tied for second place with 16 percent support each, behind former Massachusetts Gov. Mitt Romney, who has 24 percent.

Romney's numbers have remained more or less steady over the past month, as have the numbers for second- and third-tier candidates like Michele Bachmann, Newt Gingrich, Jon Huntsman, Ron Paul and Rick Santorum. But as Perry has hemorrhaged support, Cain has snatched it. The poll shows a clean transfer of support: 13 percent down for Perry, 12 percent up for Cain. Cain apparently has been the direct beneficiary of Perry's troubles.

This raises two big questions. Why is Perry slipping so badly? And why is that benefiting one candidate exclusively? Normally, when a popular candidate starts to crumble, his supporters disperse to a number of alternative candidates. It is rare to see such a neat exodus to one alternative.

The first question is easy. Perry's best poll numbers came before the September debates began, and his decline coincided neatly with the start of those debates, in which he was consistently outshone. His substantial lead in the polls started to narrow after the first debate, and with each subsequent appearance, he only made things worse.

What may yet prove to be the final nail in his coffin was a statement he made at the most recent debate, which was held in Orlando, Fla., late last month. He was asked about a law he had supported in Texas to make the children of illegal immigrants eligible for in-state tuition at state colleges and universities -- anathema to most Republicans. He refused to back down, responding, If you say that we should not educate children who have come into our state for no other reason than they've been brought there by no fault of their own, I don't think you have a heart.

Needless to say, conservatives didn't like that. Former U.S. Sen. Rick Santorum of Pennsylvania immediately jumped in, calling Perry soft on immigration and accusing him of supporting policies that I don't even think Barack Obama would be for. Even Mitt Romney, who can hardly claim ideological purity himself, said Perry was desperate to shift attention away from his liberal policies that encourage illegal immigration.

Liberal Policies?

Well, sure, extending in-state tuition benefits to the children of illegal immigrants is a liberal policy, if we want to be black and white about it. But there is more than a little bit of irony in accusing Rick Perry of liberal policies.

If you dig around in their records, you can accuse any presidential candidate of that. That's what happens when you govern: you run into reality, and you find that you can't adhere rigidly to conservative or liberal principles all the time. Perry supported in-state tuition for illegal immigrants. Romney supported health care reform in Massachusetts. It's not exactly uncommon for otherwise faithful conservatives (or liberals) to break away on one or two issues.

But the loudest voices in today's Republican electorate -- the Tea Party faction and the staunchest conservatives -- demand absolute ideological purity, which brings us to the second question: Why, when Perry lost support because voters discovered he was only 95 percent conservative, did that support go so overwhelmingly to Cain?

The answer: Cain is the only candidate left who has not yet been vetted enough to find the liberal policy that would disqualify him.

Cain has never held an elected political office, so he has had the luxury of hawking a platform with no nuance. He can say that he would throw out the entire tax code and replace it with flat taxes, eliminate all federal entitlement programs, balance the budget right away, create a no-tolerance immigration policy and ban abortion even in the case of rape or incest, because he has never had to work within a government whose executive and legislative branches are gridlocked more often than not.

In other words: he has never had to deal with political reality. He has plenty of experience with business reality and did very well in that world, and there is undoubtedly crossover between business and politics, but they are not the same, and it is absurd to claim that they are. A CEO has a lot more freedom than the president of the United States does to make unilateral decisions. A CEO doesn't have to navigate a rigid system of checks and balances.

But when you can't act unilaterally and you do have to deal with checks and balances, you learn quickly to pick your battles and compromise on the rest. And that is what the Tea Party needs to learn if it wants to stay relevant for more than a handful of election cycles: Ideological purity does not exist in the executive branch.

Purity is possible as a legislator, because you have complete control over your vote and you can cast it however you please, even if there is no chance of your position winning. That is why candidates like Bachmann and Santorum can boast rock-solid conservative records. But when your job is to craft policies that advance your agenda but can also pass Congress, dogma goes nowhere -- and that is true for conservatives and liberals alike.

Bachmann said at the recent Orlando debate that voters should not settle for anything less than a strong constitutional conservative. She thinks sound policy can be accomplished through utter rigidity: that the economy would be better if congressional Republicans had dug in their heels and refused to increase the debt ceiling under any circumstances. She does not understand that it is the gridlock itself that causes the greatest problems.

Bachmann does not understand the realities of the president's job, and neither do the Republican voters who switch their allegiance every time they spot a crumb of liberalism on a candidate's shirt.

It is impossible to be president, or even a governor, and have a perfect conservative or liberal record. Ronald Reagan couldn't do it, Franklin D. Roosevelt couldn't do it, and no president ever will.