The latest Rasmussen poll shows Mitt Romney's support among Republicans at 16 percent. That's double his share from a few months ago. It also surpasses John McCain's share of 15 percent, and puts him within shouting distance of Rudy Giuliani (25 percent).

Romney's surge isn't all that surprising. He's an able and attractive candidate, and (in part for this reason) a very well-funded one.

Romney has also benefited from some good luck. First, the latest round of activity on the immigration front appears to have hurt McCain. I suspect that McCain was always likely to take a hit over this issue and others where he stands outside the mainstream of his party. But it was important to Romney that the hit occur this spring when he, not Fred Thompson, might be the main beneficiary.

This leads to the second and key piece of Romney luck -- Fred Thompson's non-entry to date. If Thompson had entered in March when Romney was on the ropes, it might have represented a knock-out blow. Instead, Romney has had a three-month window in which to make his case as the conservative candidate in the field. Although it's far from clear that Romney has obtained that status, there can be no doubt that he's made substantial headway.