The bill now goes back to Congress, where two-thirds majorities are needed in both chambers to override the veto — which pundits have said is unlikely, given the original votes on the bill.
Votes in neither the House nor the Senate reached the two-thirds majority.
The pipeline would carry crude oil — an estimated 830,000 barrels a day — from Canadian tar sands to the Gulf Coast. Canadian energy company TransCanada would be its principal user.
