I think the difference between Clinton and McCain is that she is proposing to make up the money by taxing the windfall profits of the oil and gas companies and McCain isn't. Krugman is consistent.

Krugman's criticism of the gas tax is this:

The impression that Mr. McCain’s tax talk is all about pandering is reinforced by his proposal for a summer gas tax holiday — a measure that would, in fact, do little to help consumers, although it would boost oil industry profits.
More and more, Mr. McCain sounds like a man who will say anything to become president.

The same is true for Clinton's plan. It would do little to help consumers, because if they are going to tax the oil companies they are going to raise prices. Nevermind that in the summer the demand for gas always goes up, thereby raising prices. I've heard McCain say he wants to replenish the highway fund from the general fund. Makes perfect sense of course - we'll just take your tax money elsewhere and built up the deficit some more.

Senator Clinton is one of the smartest candidates we've seen in recent years. She knows this. She knows the Bush White House is never going to pass a windfall tax on the oil companies. She also knows that this is why the gas tax won't happen.

Billy, I'm can't believe that you agree with the gas tax idea. Surely you must see it as political pandering? It's a horrible idea. But I've gotta give her political props for it. She has set up her proposal in a way that she knows will never happen but the blame can fall right at G. W.'s feet, and in the meantime she can bash Obama over the head with it, and it will be effective in the very circles of people who she needs to keep and he needs to win over.