Senator Barack Obama is ramping up his efforts to boost turnout in November among younger voters.

Officials at MTV Networks report that he has bought commercial time on three of its networks, Comedy Central, VH1 and Spike, and that his ads could begin running on those outlets as early as Thursday.

Until this year, MTV’s networks did not accept political advertising, and Mr. Obama becomes the first presidential campaign to buy time with them, an official at MTV said.

Mr. Obama’s latest purchase of advertising adds yet another layer to what has been one of the most multi-layered, presidential advertising campaigns in history, reaching all sorts of voters with specially tailored messages on their specially tailored outlets.

Mr. Obama’s campaign confirmed it has made a “youth buy” but did not share the duration of this new rotation.