At Real Clear Politics, Sean Trende makes a well-argued case for why Mitt Romney is still positioned to win in November, despite the last three weeks of bad news, and his long-standing deficit in most polls. Like a lot of analysts, Trend gives a good amount of weight to the coming cash disparity between Romney and President Obama: “Romney and his allies will probably outspend the president heavily in the next two months.”

Yes, both sides will spend to the point of diminishing marginal returns. But the next month is when voters will be the most tuned in to the election, and a flood of ads — positive and negative — could move the dial in Romney’s direction, or at least, improve his favorability with the public.

But there are a few things to keep in mind when considering Romney’s potential ad advantage. First, campaign ad effects decay quickly — in a paper analyzing the 2000 election, political scientists at UCLA found rapid turnover for most advertisements. “Even when the persuasive effect of ads on candidate preference is large, 50 to 75 percent of the effect dissipates within the first week and almost all is gone by the end of the second week.” The effects of an ad barrage in early October could disappear by the end of the month.

Romney’s best bet would be to coordinate an assault in the final two weeks of the campaign; in a 2008 post, political scientist John Sides notes that, in 2000, voters in swing states were more than twice as likely to see a pro-Bush ad as a pro-Gore ad. Did it matter? Number crunchers estimate that it cost Al Gore as many as 4 points among undecided voters.

Can Romney build that kind of advantage over Obama? I doubt it. As it stands, Romney lacks an overwhelming cash advantage. The New York Times notes the extent to which Team Romney is hampered by limited funds: “Much of the more than $300 million the campaign reported raising this summer is earmarked for the Republican National Committee, state Republican organizations and Congressional races.” Super PACs will make up the difference, but so far, they’ve been focused on ineffective negative ads — the president is incredibly well-known to most Americans and attacks won’t do much to move opinions.

Again, none of this is to say that Romney can’t bounce back. As Trende notes, there’s a fair chance that the race will tighten and Romney will make gains in likely voter polls. But money isn’t a magic bullet; ads require particular circumstances to be effective. They require a strategy grounded in flooding the airwaves in the very last days of the campaign, an overwhelming cash advantage, and opinions of the opponent to not be locked in. At best Romney has only the first of those — so all that money is unlikely to save him.

Jamelle Bouie is a staff writer at The American Prospect. You can find his blog here.