Astate like Idaho which has no formal party registration gets its members informally, very informally, through self-definition. If you think of yourself as a Republican, or as a Democrat, then you are. And in Idaho, to judge at least from voting results, a good many more people self-define as Rs than as Ds.

That’s why you want to be very careful when you say publicly the kind of thing U.S. Representative Mike Simpson said at last weekend’s state GOP convention.

He was talking about the candidacy of Bill Sali for the other U.S. House seat; he opposed Sali in the primary but now supports him as the Republican nominee, against Democrat Larry Grant. So much was a normal pitch for party unity. Then, according to several reports, he added: “I’ve heard some talk about Republicans for Grant. There is no such thing as a Republican for Grant. They are Democrats.”

We’ve written in the last few days about a subtext of “purification” in the current Idaho Republican Party, and this may be the clearest instance of it: You have to vote not just for nearly all Republicans, but every single one, or you’re no Republican at all. Cross the line once and you’re outta here.

Is that reaction over-sensitive, the misreading of an independent viewpoint? Well, consider the testimony of Bubblehead, a blogger, a retired submarine officer and a self-described lifel0ng Republican who has voted mostly Republican and never Democratic for president. [The point came via Red State Rebels.] After meeting and talking with both Sali and Grant, he decided to support Grant.

After hearing about Simpson’s line, Bubblehead responded: “I’ll be honest – this upsets me quite a bit. I feel I’ve done enough for my country to be accepted as a member of either one of the two main political parties, no matter who I happen to vote for in one election. And anyway — who is Mike Simpson to throw me out of my own party?” After which he goes on to rant about Simpson and the party. The seeds of a larger-purpose walkout may have been planted.

No one knows yet, or will for a while, what sort of crossover vote Grant may attract. But if it is substantial, Simpson – and some other Republican leaders – may wish the congressman could take his words back.