Advertising blitz seeks to sway U.S. debt debate

* 'Unique situation'

* Millions collectively spent on ads

By Kim Dixon

WASHINGTON, July 29 (Reuters) - As U.S. lawmakers scramble to clinch a debt-ceiling deal before a Tuesday deadline, interest groups are blitzing the airwaves to sway members of Congress still sitting on the fence.

From the pro-business Club For Growth to national labor unions, outside interests are spending millions to try to influence a fractious debate in Congress over how to deal with the country's yawning budget deficit and whether to raise the government's debt limit.

With any legislation expected to pass by only a razor-thin margin, every lawmaker's vote will likely count.

The looming deadline gives the ads more immediacy and potentially adds to their influence, political communication experts said.

"This is a unique situation because of the ticking clock and this kind of brinkmanship," said Michael Cornfield, a George Washington University professor. "The only parallel I can think of is the last 72 hours during an election, and usually then it is not as close."

The House of Representatives began debate on Friday on a Republican-backed bill that slashes spending over a decade to raise the debt ceiling in two stages.

Senate Democrats are expected to kill that plan, setting the stage for a potential compromise. The government's $14.3 trillion borrowing limit must be lifted by Tuesday or the country will risk an unprecedented default and downgrade of its credit rating.

ADS FROM LEFT AND RIGHT

An ad from American Crossroads, which is committed to spending $20 million in ads in coming months to unseat President Barack Obama, features a small girl about to be crushed with shipping containers hanging from a crane, one labeled "$14 trillion debt."

The 30-second ad released this week features a breathy woman's voice and ominous music, saying, "America's economy is hanging by a thread ... it's time to take away President Obama's blank check."

The ads will run in Colorado, Iowa, North Carolina, New Mexico, Nevada and Virginia -- states seen as up for grabs in next year's presidential election.

American Crossroads, a well-funded conservative outside spending group conceived by Republican strategist Karl Rove, spent $70 million in the 2010 congressional elections to help unseat Democrats.

Ads from groups traditionally allied with the Democratic Party, including the Service Employees International Union and other labor organizations, have been blasting the airwaves since last week, heaping blame on Republicans if Social Security and veterans' checks are halted.

The SEIU and three other groups launched 30-second television spots in eight states on Friday. A spokesman says they spent "six figures" for the eight ads, after running four last week.

One juxtaposes a picture of a kindly grandfather and a soldier hugging his grinning child with corporate suits getting out of a limousine and a private jet.

The ad urges Republicans "to stop holding the interests of ordinary Americans hostage."

The Club for Growth also made a "six-figure" ad buy in Indiana and a slightly smaller one in Utah this month, putting pressure on lawmakers to demand deep spending cuts and a balanced budget amendment.

Spending on political advertising, particularly by groups not coordinated with any one politician, is expected to explode heading into the 2012 election after several court decisions easing contribution limits and donor disclosure rules. (Additional reporting by Patricia Zengerle; Editing by Peter Cooney)