Today, tomorrow and the weekend are the last chance McCain has to get back into this race. To understand why, let’s briefly recap what’s happened since the conventions.

McCain left the back-to-back conventions with a lead for the first time in the presidential race. His strength was fueled by his designation of Sarah Palin and the enthusiasm she aroused in the Republican base. The Democrats, caught off guard, attacked her viciously, which only reinforced the perception of a woman being victimized by a sexist left.

Then McCain lost the first two debates. Regardless of what observers think happened (we thought he lost the first and won the second), the fact is that his poll numbers deteriorated after each performance. The financial crisis extinguished any lead McCain may have had, exacerbated by what must be seen, in hindsight, as the biggest mistake of his campaign — his decision to “suspend” his campaign and return to Washington to work on the bailout. McCain’s subsequent docile support for the package, earmarks and all, destroyed the perception he had carefully built during his convention and subsequently: of independence from Bush and of a populist antipathy to greedy Wall Street barons.

As a result of the debates, the crisis and McCain’s ineptitude, Obama surged to a lead of seven to nine points around Oct. 13.

Then, amazingly, McCain won the third debate. Using a populist appeal and fighting hard, he echoed the complaints of Joe the Plumber against Obama’s plan to “spread the wealth around.” From Wednesday, Oct. 15 through Saturday the 18th, McCain sliced the Obama lead to five points — an average of all the polling.

But the effect of the debate began to wear off on Sunday, Oct. 19 and McCain began to fall back again. By now, Obama has an average lead of 7.2 points, according to (The AP poll showing a dead heat is the only one to show such data, unfortunately.)

Much of Obama’s rise this week has been due to his domination of paid advertising. With his coffers swollen by $150 million raised in September, he can buy any available advertising time. But McCain has considerable resources at his disposal as well. For the week that started this past Tuesday, Oct. 21, McCain is running about four ads to every five that Obama airs in swing states.

(Technically, McCain is airing about 2,500 household rating points per week against Obama’s 3,300. One household rating point means that 1 percent of the households are watching the ad. So 2,500 points in a given week suggests that each household is seeing about 25 ads per week, or about four each day. While Obama is spending more money, the difference between seeing four McCain ads and five Obama spots each day is not significant.)

And, starting on Monday of this week, McCain began running an attack ad specifically aimed at the issues first raised by Joe the Plumber. Showing footage of his chat with Obama, the McCain ad hammers home the tax issue, noting that Obama’s so-called tax cut for 100 million Americans is really a welfare check for the half of them who pay no taxes. The ad is hard-hitting and effective. More importantly, it is running in the clear without other McCain media to clutter up the message.

So McCain has his best ad on his best issue with as close to financial parity with Obama as he is likely to get. If, in these circumstances, he cannot gain ground during the next few days, the race will be functionally over. But if the Republicans can get traction, it could become a competitive contest again.

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