Behind the battle is the prospect that the cable modem, with its sprawling, web-friendly bandwith and on-all-the-time convenience, is coming to the mainstream in a few years, and that a one-stop port for Internet, cable and phone service will soon be as ubiquitous in American homes as the traditional telephone jack. In the race for cable lines, AT&T is out front, and MCI Worldcom is understandably worried. They're now mulling joining forces with jilted Comcast to win back MediaOne's hand. Other interested parties include AOL, Microsoft and current No. 1 cable company Time Warner (behemoth parent to TIME Daily). But Time Warner would just as soon see AT&T go home with the bride, which would extricate it from a sticky partnership deal with MediaOne that Comcast planned to make Time Warner honor. As for AOL and Microsoft, both Net companies have the cash to make a move. But they're starting from scratch in the cable game and may want to simply wait this war out, then cut a partnership deal with the winner. So far the favorite's AT&T. But by Thursday, the deadline for Comcast's right of first refusal, someone might decide to play spoiler.