“Nice middle class you got here,” said Mitch McConnell, the Senate minority leader. “It would be a shame if something happened to it.”

O.K., he didn’t actually say that. But he might as well have, because that’s what the current confrontation over taxes amounts to. Mr. McConnell, who was self-righteously denouncing the budget deficit just the other day, now wants to blow that deficit up with big tax cuts for the rich. But he doesn’t have the votes. So he’s trying to get what he wants by pointing a gun at the heads of middle-class families, threatening to force a jump in their taxes unless he gets paid off with hugely expensive tax breaks for the wealthy.

Most discussion of the tax fight focuses either on the economics or on the politics  both of which suggest that Democrats should hang tough, for their own sakes as well as that of the country. But there’s an even bigger issue here  namely, the question of what constitutes acceptable behavior in American political life. Politics ain’t beanbag, but there’s a difference between playing hardball and engaging in outright extortion, which is what Mr. McConnell is now doing. And if he succeeds, it will set a disastrous precedent.

How did we get to this point? The proximate answer lies in the tactics the Bush administration used to push through tax cuts. The deeper answer lies in the radicalization of the Republican Party, its transformation into a movement willing to put the economy and the nation at risk for the sake of partisan victory.