What's a RINO? How the Republican purity test took an odd turn.

Something peculiar happened in D.C. this week – beyond the peculiarities that have become standard operating procedure in the Capitol, at least.

Republicans proposed to cooperate with Democrats and no one made a much of a peep.

The prospect of Republicans and Democrats cooperating, of course, has become so unusual as to seem fantastical – as though "bipartisanship" last roamed the earth with the wooly mammoths and men in mukluks. 

Indeed, one seemingly ironclad rule in a House of Representatives that apparently has no ironclad rules is that Speaker John Boehner (R) of Ohio will not bring a bill to the floor – even if he supports it, and even if it could pass – unless it has majority support among Republicans. For a man trying to hold on to his job, this "Hastert Rule" (named for former House Speaker Dennis Hastert) makes perfect sense. For parties trying to meet in the political middle, it is a death warrant. [Editor's note: The original version misidentified the "Hastert Rule."]

Yet this week, Sen. Rand Paul (R) of Kentucky and Sen. Patrick Leahy (D) of Vermont joined forces on a bill that would promote a top law-enforcement priority of President Obama. Yes, that Rand Paul.

The bill would help ease mandatory minimum sentences for nonviolent drug offenders. The rationale, from Senator Paul's perspective, is simple: The United States is spending too much money to incarcerate people who are not a serious threat to society. In other words, it's all about big government and American liberty. And some Democrats – the most liberal kind, actually – happen to agree for their own reasons.

Presto! A bipartisan coalition unlikely to spawn any tea party primary challenges.

So if one of the flag-bearers of the new Republican Party can find common ground with one of the most liberal members of the Senate to forward an item on President Obama's agenda, then what is a RINO (Republican In Name Only)?

The answer, of course, is that it is what the voters decide that it is.

A look at Republican voters leaves little doubt as to what are core Republican principles. A poll (admittedly by Democratic pollster Stan Greenberg) found that 75 percent of the Republican Party self-identifies as Evangelicals (30 percent), tea partyers (22 percent), or religiously observant non-Evangelicals (17 percent). Only the remaining quarter self-identify as moderates.

Law and order? Military spending? The new Republican Party can manage a bit of ideological diversity there. But budgets and taxes? Consider that the "Boehner rule" demands that any raise in the debt ceiling be offset by equal spending cuts. 

The 1990s saw the rise of the religious right that flipped the South red. The 2010 midterms saw an influx of the small-government libertarians weaned on former House Speaker Newt Gingrich's government shutdowns. Together, they are the coalition that rules the modern Republican Party.

If you are one of them, it seems, even cozying up to Attorney General Eric Holder can be forgiven. If you are not, then prepare for a primary challenge.