Barack Obama's 30-minute primetime TV ad was seen in 21.7% of households in the top 56 local media markets, according to Nielsen. By comparison, Between 34.7% and 42.0% of households watched the three debates between Obama and John McCain. More than 31% of households in Baltimore watched the telecast, the highest percentage for any market.

UPDATE: Nielsen reports that 33.5 million people watched Obama's ad across seven stations between 8 and 8:30 ET last night.