About a fifth of U.S. households watching television Wednesday saw Barack ObamaBarack Hussein ObamaBiden hits 59 percent approval rating in Pew poll Cuba readies for life without Castro Biden can make history on nuclear arms reductions MORE's 30-minute commercial, according to Nielsen's ratings.

The ad got a 21.7 rating in the top 56 local television markets. The rating takes into account only those homes watching television at the time.

The rating is higher than Ross Perot's extended commercial in 1996, which received a 16.8 rating. The ratings for the three presidential debates ranged from about 35 to 42.