NEW YORK (AP) — Savannah Guthrie filed a hard-hitting report on NBC's "Nightly News" recently questioning the truth of some McCain-Palin campaign statements on the "bridge to nowhere" and Sarah Palin's foreign travel. Guthrie's story challenged the claims in one of McCain's TV commercials.

A few minutes later, "Nightly News" ran one of McCain's campaign spots during a commercial break.

It made for an odd juxtaposition, one not unique to NBC. Evening newscasts on ABC and CBS also ran McCain commercials during the past few weeks. Ads supporting a candidate for office are nothing new to local news viewers, but were once frowned upon — if not banned outright — from the networks' flagship newscasts.

The worry was they could get in the way of the journalism. Though the practice may have changed, some experts feel the concerns haven't.

"It can be confusing and, more importantly, distracting to the news coverage," said Bob Steele, a DePauw University professor and scholar for journalism values at the Poynter Institute.

The networks don't consider it much of an issue since ads frequently flood local news programmes.

The 20 million-plus people who watch Brian Williams, Katie Couric or Charles Gibson in the evening are prime targets for political candidates. Viewers are primarily older, the kind of people most likely to vote and, since they're watching the news, are obviously engaged in what's going on in the world.

This is the first presidential campaign since Bob Dole's in 1996 that candidates have broadcast TV commercials seen across the country at the same time, said Evan Tracey, head of TNS Media Intelligence/Campaign Media Analysis Group, a firm that tracks ads. The preference is for local ads, which are cheaper and allow campaigns to reach specific communities in battleground states with messages that resound there.

Small national ad buys are back in vogue this year because they're attention-getting; Barack Obama advertised on NBC during the Olympics. More states are also considered up for grabs, tipping the balance in making the national ad buys cost-effective, Tracey said.

McCain has bought national advertising time during game shows and soap operas, targeting older women who are at home during the day, he said.

Networks are also mindful of how certain local stations hit the jackpot every four years when they have the good fortune of being in battleground states. In a tough economy, it's hard to turn down potential revenue; the price of a 30-second spot during the evening newscasts fluctuated between $40,000 and $55,000 earlier this year, according to Horizon Media.

Representatives from ABC, CBS and NBC said it wasn't a change in policy to accept these ads for the evening news. To a certain extent, there wasn't a need for a policy because the issue didn't come up.

That may be true, but some veteran news executives, including former "World News Tonight" executive producer Tom Bettag, said they believed there was an understanding — either spoken or unspoken — that they weren't considered appropriate for the evening news.

Joseph Abruzzese, former CBS ad sales chief who worked there for more than 20 years through 2002, said that candidate commercials weren't taken for news programmes to avoid confusion with editorial content. He said he was "very surprised" to see a McCain commercial pop up on the "CBS Evening News."

Although ABC permits the spots during "World News," the network makes a special effort to place them in commercial breaks several minutes away from political stories, spokeswoman Cathie Levine said. The candidate pitches are also placed in the middle of commercial breaks, with ads both before and after them, to further differentiate them from editorial product, she said.

NBC's "Nightly News" ran McCain ads on four of five nights the week of September 15. The commercials essentially showed the GOP candidate commenting on things fresh in the news, like the bubbling economic crisis. Obama advertised on the NBC newscast in the spring.

"We're completely confident that our viewers can tell the difference between our news coverage and a campaign ad," network spokeswoman Lauren Kapp said.

Ken Goldstein, director of the University of Wisconsin Advertising Project, which follows ad spending, said he doubted these evening news commercials would have any influence on news coverage or be confusing to viewers.

"I am so used to the world where political ads are on during newscasts," Goldstein said. "It is so obvious on local programmes, I see no reason not to do it on a national level.'

Evening newscasts are flooded with prescription drug advertising yet still do a lot of health coverage. Andrew Tyndall, a consultant who studies news content, believes the situations are similar.

But Steele said he could see situations where the advertising could get in the way of the journalism.

Newscasts are increasingly parsing political commercials to tell viewers whether its claims are true, false or misleading. If the same newcast runs a political spot, could viewers assume that its content has been vetted by the news division, too?

Steele wondered whether a newscast would take on an unwanted whiff of partisanship if one candidate is advertising heavily and the person's opponent isn't. "I'm old-fashioned enough to say my preference is to have candidate ads outside of the newscast," the journalism ethics expert said.