There will be no summer idyll for voters in swing states, who will be deluged with tens of millions of dollars in political ads over the next month as part of an intensifying broadcast war through the Olympic Games.

In addition to spots from President Obama and Republican challenger Mitt Romney, many ads will be aired by independent nonprofit groups that are not required to reveal who is funding them. The surge comes ahead of a legal deadline that could force some groups to begin identifying their donors.

Crossroads GPS, a conservative nonprofit group co-founded by former George W. Bush adviser Karl Rove, announced Friday that it was purchasing $25 million worth of anti-Obama ads in swing states pegged to June’s disappointing unemployment report. The campaign starts Tuesday and will run through early August in nine swing states: Colorado, Florida, Iowa, Michigan, North Carolina, New Hampshire, Nevada, Ohio and Virginia.

“Barack Obama’s got lots of excuses for the bad economy,” a narrator says in the Crossroads ad. “But Obama never blames Washington’s wild spending and skyrocketing debt.”

The Crossroads buy is part of a blizzard of ad spending planned by the presidential campaigns and their allies over the next month, most of it concentrated in about a dozen battleground states that are expected to hold the key to the election in November.

The Obama campaign has reserved about $25 million worth of television and radio spots in July, continuing the heavy pace it began in May, according to advertising trackers. Those ads come on top of a steady stream of spots from Democratic allies including the Priorities USA Action super PAC, which has spent weeks hammering on Romney’s career as a corporate buyout specialist.

The Romney campaign has not announced its full plans for July, but has been spending about $750,000 a day on average so far this month. One recent spot features footage of Hillary Rodham Clinton, now secretary of state, criticizing Obama during the fierce 2008 Democratic primary.

Then there are the Olympic Games, broadcast from London beginning July 27, which have attracted at least $13 million so far in connection with the presidential race. Obama has bought about $5.5 million worth of airtime on U.S. networks, while Restore Our Future, a pro-Romney super PAC, says it will spend $7.2 million.

When all is said and done, the two presidential candidates, their parties and independent groups could spend $100 million or more on advertising over the next month, nearly all of it trained on a narrow group of swing states.

Ken Goldstein, president of the Campaign Media Analysis Group, which tracks ad spending, said the volume of political ads is unusually high for summertime, which has traditionally marked a lull before the party conventions in a presidential election year.

“It’s incredibly intense and incredibly concentrated. There does not appear to be much ebbing and flowing that’s going to go on for the next couple months,” Goldstein said. “There’s lots of players, there’s lots of money and you have a competitive election. It’s a perfect storm.”

Much of the ad spending related to the presidential race comes in the form of “issue ads” from conservative groups like Crossroads GPS or Americans for Prosperity, which are organized as tax-exempt “social welfare” organizations that do not have to disclose their donors. Issue ads do not specifically advocate voting for or against a candidate, although most average viewers would be hard-pressed to tell the difference.

But a federal judge ruled earlier this year that such groups would have to reveal some or all of their donors if they run issue ads before the conventions or general election — a clock that starts ticking in early August.

Campaign-finance experts say the disclosure deadline could help explain part of the surge in July advertising, although several advocacy groups said they were still assessing the ruling and had not factored it into their decisions. Some of the groups could end up following the lead of the U.S. Chamber of Commerce, which has said it will evade the ruling by abandoning issue ads in favor of spots directly endorsing or opposing congressional candidates.

Rachel Weiner contributed to this report.