A commission member said that the new agreement on the vice-presidential debate was reached late morning Saturday. It calls for shorter blocks of candidate statements and open discussion than at the presidential debates.

McCain advisers said they were only somewhat concerned about Ms. Palin’s debating skills compared with those of Mr. Biden, who has served six terms in the Senate, or about his chances of tripping her up. Instead, they say, they wanted Ms. Palin to have opportunities to present Mr. McCain’s positions, rather than spending time talking about her own experience or playing defense.