You generally need to wait until you know that the individual is skewered and then go for the kill. Former shadow home secretary David Davis is a past master at the art.
And this afternoon, George Osborne has opted to turn the screw on Lord Myners, the Treasury minister who has previously claimed to Parliament (in the House of Lords and through evidence to the Treasury select committee) that he did not know about the details of Sir Fred Goodwin's RBS pension.
Yet today Sir Tom McKillop, the former RBS chairman, has written this letter to the Treasury select committee in which he asserts that Lord Myners had not "accurately represented" what he knew.
The senior Tory on the committee, Michael Fallon, has already said today that "the decent thing" for Lord Myners to do would be to resign. And George Osborne now senses blood.
In a Commons debate on the economy, he invited Chancellor Alistair Darling to agree that:
The Chancellor's reply hardly demonstrated full support for his ministerial colleague:
That hardly amounts to the "full confidence" a Downing Street spokesman said that the Prime Minister had in Lord Myners at the end of February.