US News and World Report's James Pethokoukis makes the case for Romney:

Why are so many economic conservatives still iffy about John McCain? Sure, he wants to cut taxes and expand free trade, but he's still pushing the regulatory leviathan that is cap-and-trade and still bashes unpopular industries like oil and pharmaceuticals. But it's more than that. McCain, whatever his 2008 campaign agenda, has yet to become a forceful and persuasive advocate for smaller government and freer markets....

Once again there is talk of dramatic tax increases in the name of fairness, government intervention in key economic sectors, and the evils of business (hedge funds, healthcare, energy). Could Mitt Romney, a guy who might be right at the top of McCain's short list for vice president, be "The Man Who Talked Back" for a new era as veep? Many free-market folks I chat with sure hope so.

That ellipses is actually a big jump so be sure to read the whole thing. Essentially Pethokoukis' point is that with everything moving in the direction of bigger government, tax hikes, etc., McCain needs a free-market voice, since he can't offer it himself.

And it's sound advice. It's just unfortunate that Romney is McCain's best choice here. As a candidate with oodles of cash, Romney barely made a dent with voters. Conservatives only jumped on his candidacy after it was clear that there was no one else. Not exactly a ringing endorsement. Put simply, Romney's primary run didn't do much to convince skeptics that he is a good candidate with wide appeal.