IN 12 YEARS on Capitol Hill, Sen. Mark R. Warner (D-Va.) has staked a legitimate claim as one of the more effective, constructive and genuinely bipartisan lawmakers in Congress. In a place where grandstanding, pandering and interparty venom are nearly normal, Mr. Warner is frequently the grown-up in the room, a measured and substantive Democrat who has forged partnerships with key Republican colleagues. Virginians would do well to reelect him this year to a third term in the Senate, and we endorse him.