Google and Comcast are in serious discussions with Time Warner about buying a minority stake in America Online, according to people familiar with the situation.

The negotiations focus on AOL’s Web portal, rather than its dial-up Internet access business, which is suffering a steady decline of subscribers. Google and Comcast are hoping a stake in AOL’s portal will allow them to tap AOL content, such as AOL’s successful Webcast of the Live8 conference this summer, to draw more consumers to their online services and related advertising.

How closely Google and Comcast are aligned in the talks isn’t clear. Google approached Comcast about participating in a bid for AOL last week, according to a person familiar with the matter. But Google may end up making a bid on its own, another person said.

The deal would put a value on AOL,as a whole, of about $20 billion, the people said. Any bid would be worth considerably less, however, as AOL’s dial-up operations generate lots of cash and would account for much of its value.

If consummated, a combination of AOL, Google, and Comcast could create a potent challenge to Yahoo, which is currently the Internet industry leader in terms of audience size and advertising revenue.

Currently AOL and Google have a close relationship. AOL uses Google’s search-engine technology, and Google gives AOL a cut of the advertising revenue generated by AOL customers. Last year, Google generated $300 million in revenue for AOL.

The negotiations also present complications for Microsoft, which has been talking for months with Time Warner about combining their Internet operations.

A spokesman for Time Warner, Ed Adler, declined to comment. Google wa not immediately available for comment. Asked about any talks with AOL last week, Google said in a statement, “Google and AOL have a healthy global partnership, and AOL remains a valued partner. Your inquiry is about rumored conversations and we’re not able to respond to questions of this type.”