When the back-to-back party conventions get underway Monday, the usual hordes of partisans in funny hats, political journalists, and memorabilia merchants will be joined by one group you don't often see in the political arena—corporations.

The Democratic and Republican confabs in Denver and Minneapolis-St. Paul, respectively, will attract 16 "corporate providers." By law, corporations can't contribute directly to the political process and can't, as organizations, give to the presidential candidates. But they can pony up funds for the conventions, this year estimated to cost $50 million apiece, $16 million coming from taxpayers and the rest from the private sector.

Not every advertiser will be writing checks to the political parties; most have signed agreements to provide services and manpower in addition to cash. The tab for sponsorship: up to $5 million, in the case of Comcast's deal to be the cable TV and video-on-demand provider to the Democratic National Committee.

General Motors, Microsoft, and other advertisers will appear at both conventions, but a few companies have chosen sides, and not always the ones their C.E.O.'s endorse.

AT&T, for example, made a monetary contribution to both conventions but is only providing services and devices to the Democratic convention in Denver. This despite the fact C.E.O. Randall Stephenson gave $2,300 to John McCain's Republican presidential campaign.

And YouTube's C.E.O., Chad Hurley, gave the same amount to Barack Obama's Democratic campaign even though YouTube will only be a presence at the Republican convention in Minneapolis. As early as July, the online video provider began stirring up buzz for the G.O.P. bash as it asked Americans to post video responses to the question "Why are you a Republican in 2008?" The winner was awarded a day on the trail with McCain's campaign and a ticket to the convention, where the winning video will be shown. While the Republican National Convention Committee called the contest a way to "empower supporters," it was also clearly a marketing technique for YouTube.

What does a corporation get for its cash? First and foremost, exposure. General Motors has had a presence at both conventions since 1980, but this year the company will show off its hybrids and flex-fuel vehicles. When the conventions are over, the 745 G.M. cars that were used will be sent to dealerships or auctioned off.

Microsoft will plug its new Silverlight platform, an application that will deliver video coverage of the convention in high definition. (It's the same application NBC used to broadcast the 2008 Beijing Olympics.) Microsoft will also provide attendees with large touchscreen kiosks that dispense the latest convention and local city information—transportation routes, hotel locations, restaurant guides, and meeting schedules. It's an effort by Microsoft to make the convention experience user-friendlier.

Analysts say a company's image shouldn't be jeopardized by being tied to a major political event like the conventions. Consumers pay little attention to what a corporation (or its officers) is doing on a personal or political-action committee basis, says branding consultant Robert Passikoff. "The only time it matters is when it's a human brand like Martha Stewart," he says.