If the Democrats persist in stonewalling the impeachment of George Bush and Richard Cheney, they invite a humiliating defeat in the presidential election this fall.

For more than a year, the Democrats have gamed the system of Constitutional democracy, refusing to impeach—“It would be too divisive”—in order to assure a Democratic victory in 2008. But the year produced some surprises, and now their scheme stands an excellent chance of backfiring.

A year ago, John McCain’s candidacy languished. Today he is the Republican nominee and a formidable opponent. A year ago, Hillary Clinton’s nomination was “inevitable. Today she clings to a miniscule possibility of succeeding in her vitriolic campaign against Barack Obama, who holds a commanding lead in pledged delegates.

The savage nature of the contest is polarizing the Democratic party—to Mr. McCain’s considerable benefit.

And the Democrats’ refusal to impeach has now become a grave liability: it adds enormously to Mr. McCain’s advantage.