With the steady decline in Rudy Giuliani’s poll numbers over the last six weeks has come the steady rise of John McCain’s, especially the latter’s vertiginous upward swing in New Hampshire — where, according to a new poll, McCain has taken the lead over Mitt Romney in the primary that will be held in six days. In the latest Pew poll of Republicans nationally, McCain and Giuliani are now tied for the lead, as they basically were during 2005 and the beginning of 2006. In those polls, they tended to split about 60 percent of the primary vote; in the latest, the number is closer to 40.

There was always a most interesting aspect to these numbers, since, for all intents and purposes, the candidacies of Giuliani and McCain are one and the same — a pitch to be the president best suited to fighting the war on terror based on strong leadership skills and personal heroism (I am not here comparing in any way Giuliani’s conduct on 9/11 with McCain’s years in the Hanoi Hilton, just noting the logic in the minds of voters). From the moment George W. Bush was reelected, the Republican voting public was offering signs that its ideal candidate would be a war candidate and that it would line up easily and quickly behind the right one. Unfortunately, McCain was saddled with a bunch of liabilities owing to his conduct as a presidential candidate in 2000, his peculiar votes against the Bush tax cuts in 2001 and 2003, and his full-throated support of immigration reform in a party with increasingly nativist tastes.

Giuliani’s candidacy was made possible by McCain’s weaknesses, and when the McCain campaign seemed to implode in the middle of 2006 (running out of money, firing a campaign manager and longtime aides), he seemed poised to benefit strongly from it — as voters jumped off the McCain bandwagon, it would only make sense for them to jump on Giuliani’s. And they did. But it turned out Giuliani hit his rough patch in late November and early December, with unfavorable news stories reminding people of his complex marital history and the poor behavior of some of his allies. And so it is McCain who, as New Hampshire approaches, is benefiting from Giuliani’s weakness.

McCain’s shot at becoming the Republican nominee seems dependent on Giuliani fading very fast. And Giuliani’s shot seems dependent on McCain’s surge in New Hampshire proving to be a single-state phenomenon.