If you've looked at Parents.com recently, you may have run into Laura Bush.

"Education is my passion. And the president's too," the first lady said in an ad stretched across the top of the magazine's Web site. "Let me explain why."

And in the ad she did -- in a full-motion 21/2-minute video.

"I'm Laura Bush," she said. "From Day One, the president has been a friend to parents and teachers who want to improve America's schools."

Meanwhile, Democratic challenger John F. Kerry was plying readers for cash on the New York Times Web site. "If you want Kerry to win in November we need your help today," his ad said, alongside a picture of the senator from Massachusetts. Click on his ad and you are routed to the online donation page of his campaign's site.

It is the latest in what has become an escalating presidential ad war online. The candidates -- no longer content with building elaborate Web sites or compiling massive e-mail lists -- are taking their messages to scores of other sites, in hopes of reaching the millions of Americans who use the Internet every day.

The Bush campaign's education spot -- its first foray into online advertising this election cycle -- has been playing on about 50 sites, each chosen with an eye toward its audience's demographics. Many cater to women with children: In addition to the site of Parents magazine, there are those of Parenting, Better Homes and Gardens, InStyle, Ladies' Home Journal, and the Food Network.

Other sites reach voters in critical swing states and are operated by newspapers, including the Columbus Dispatch, Akron Beacon Journal, Miami Herald, Tampa Tribune and St. Louis Post-Dispatch.

Kerry has placed ads on at least 100 Web sites, according to the independent research firm Nielsen/NetRatings and research by The Washington Post. Many are newspaper sites, such as those of USA Today and the Los Angeles Times. Others have little to do with politics or news, but enjoy sizable followings, including the Onion and Billboard.com. Kerry's campaign also has placed ads on some liberal blogs and purchased at least one ad on the search engine Google that appears whenever users search for information using the keywords "the economy."

Kerry's campaign -- like the president's -- declined to say how much it has spent on the ads. But both said the spots are becoming an important way to reach voters. "We're continuing to try innovative new ways to reach people," said Michael Meehan, a spokesman for Kerry. "This is certainly one of the arrows in our quiver in our online fundraising strategy."

"Our campaign will look for as many different ways to communicate President Bush's positive message as possible," campaign spokesman Scott Stanzel said.

It is a sharp contrast from the 2000 election, when the presidential contenders only dabbled in online advertising. Most concluded that their ads were a bust. "It didn't make financial sense to advertise online," said Max Fose, an Internet strategist for the presidential campaign of Sen. John McCain (R-Ariz.). "It was too expensive to get the results you needed to make it worthwhile."

Since then, however, millions more people have gone online. High-speed Internet connections have become increasingly common -- about 68 million Americans have "broadband" connections at either home or work, according to a study by the Pew Internet & American Life Project. The private sector, which shrank from online advertising in the wake of the dot-com bust, has returned in large numbers -- paving the way for risk-averse politicians.

Internet consultants outside the presidential campaigns said candidates have slowly but steadily become sold on the ads. More have realized that while millions of people are online every day, relatively few visit the campaign sites or sign up for their e-mails. The ads not only reach those people but are also particularly effective at reaching them at work, experts said. The ads have become cheaper and more sophisticated -- incorporating video and animation -- and can be targeted to ever more narrow segments of the public.

Candidates can buy ads, for instance, that only appear in certain sections of Web sites. During the Iowa caucuses, the Kerry campaign ran ads in the "help wanted" section of the Des Moines Register's Web site. The appeals can also be directed at users who fit certain demographic and geographic profiles. The Washington Post's site, washingtonpost.com, which Kerry also has advertised on, requires readers to provide their Zip code, gender, income level and other personal information. That data is used to target advertising to individual users -- so much so that two readers viewing the same page from different computers may see different ads.

Kerry has launched a number of ads, most of which focus on closing his fundraising gap with Bush. "If you want Bush out of the White House we need your help today," said one ad, alongside an unflattering picture of Bush. Another animated ad depicts Bush as a king, sitting on a pile of gold. "Bush is attacking John Kerry with a mountain of money," the ad says. "Your $25 gift now can put us over the top and help topple Bush!" His campaign declined to say how much it has raised from the ads.

The candidates' allies are also buying ads. America Coming Together, an independent Democratic group, has begun advertising on several liberal blogs and Google, and is preparing to run ads on the search engine Yahoo and several national newspapers' Web sites. Wes Boyd, co-founder of the liberal advocacy group MoveOn.org, said his organization will probably begin advertising online this year. The anti-tax Club for Growth is also considering buying ads, while the Democratic National Committee has begun experimenting with Google ads.

The Republican National Committee has been particularly aggressive, posting ads on about 1,500 sites each of the past four months. "Show your support for the president's success and learn more about the future," said one. The spots have run on a wide range of sites, including AutoTrader.com and the Original Hip-Hop Lyrics Archive.

"We're working to reach out to as broad an audience as possible," said Christine Iverson, a spokeswoman for the committee.

The party declined to estimate how much it has spent on the effort. But its campaign dwarfed those of both Bush and Kerry during the first four months of the year, according to Nielsen/NetRatings. The group measures ad traffic in terms of "impressions," which is the number of times an ad appears on a computer screen. If you see an ad somewhere -- that is one impression. If you hit the "refresh" button and see it again, that's another impression.

The firm reported that over those four months, the RNC racked up more than 550 million impressions. Kerry's campaign drummed up more than 60 million, while Bush -- who launched his education ad this month, after the study had ended -- barely registered.

Those totals are modest compared with what many companies spent on online ads. AT&T Wireless, one of the largest advertisers online, ran up more than 12 billion impressions over that same period, according to Nielsen/NetRatings. Moreover, while the presidential campaigns declined to say how much they are spending on the ads, outside experts said it is probably a fraction of what they spend on television ads.

But they predicted that as the election nears, the ads will become more common. "It's as near to certainty as you have in the world of the Internet," said Michael Cornfield, a political scientist at George Washington University. "The closeness of the race is not deterring innovation -- it seems to be spurring it."