All politics may be local, but political advertising on cable television increasingly is going national.

The Republican Party’s decision last week to buy national ad time on cable networks for the first time in an off-year election followed similar purchases from advocacy groups supporting both parties. The national buys augment the heavy advertising that candidates, parties and advocacy groups conduct on local cable systems and TV stations.

Networks such as Fox News Channel and CNN are poised to benefit as local advertising time gets scarce in markets where races are tight for the Nov. 7 election. The chance that the Republican Party may lose control of both houses of Congress has raised the stakes in many districts, leading to a run on local ad time in states including Ohio, Indiana, Pennsylvania and Kentucky.

So far, the national cable advertising is a trickle, perhaps around $3 million, according to Evan Tracey, chief operating officer of TNS Media Intelligence’s Campaign Media Analysis Group. The national advertising may increase in future races, Mr. Tracey said, as parties seek to strengthen brand loyalty among their supporters.

The tight races in this off-year election may push total political advertising as high as $1.7 billion, the same amount spent in the last presidential election cycle, he said.

Fox News and CNN didn’t return calls seeking comment on how much national political adverting they’ve collected.

Both the Democratic-oriented September Fund and the GOP-oriented Progress for America have launched cable news ads. One of the spots from the September fund features people talking to a bush, asking, “So what’s our exit strategy from Iraq? Why do our soldiers keep dying? Why is the whole world mad at us?” That ad ends with a “Vote for a new Congress” graphic.

Progress for America’s spots talk about the war on terror. “These people want to kill us,” it says, picturing terrorists. “Many seem to have forgotten the evil that happened only five years ago. They would cut and run in the Middle East, leaving al Qaeda to attack us again. The war on terror is a war for our country’s freedom, security and survival.”

Harold Ickes, the former deputy chief of staff under President Clinton who now heads the September Fund, said spending on national cable buys has increased as local time becomes harder to find.

“It’s a combination of factors, but it’s partly what is available,” Mr. Ickes said. “Our time buyer and pollsters tell us cable is very cost-efficient.”

The group’s advertising focuses attention on what is at stake in the election, he said. “We want to help refocus on national issues and conditions and what is at issue like Iraq and the economy,” he said.

Mr. Ickes’ group, which is trying to raise $10 million to push for a change in control of Congress, is still running most of its ads locally. The September Fund asked eight Democratic media shops to produce 12 ads. The “bush” ad was put together by A-political, the shop set up by former BBDO creative Jimmy Siegel to do work for Eliott Spitzer’s gubernatorial campaign in New York. The September Fund declined to disclose how it is allocating its ad spending.