Democrats lead in September advertising for House races as California and New York dominate

 Democrats outspent Republicans on advertising by a combined total of more than $31 million across 20 of the most competitive US House races in September, ad data shows, and held the edge in 17 of those contests.

Each of the races drew between about $5 to $10 million in ad spending during the month, with two toss-up contests in Alaska and Pennsylvania topping $10 million, according to data from the ad tracking firm AdImpact.

The list of 20 races that saw the most ad spending last month stretches across the country, with five states hosting a pair of top contests, including California, New York, Pennsylvania, Michigan, and Arizona.

And overall, including races not on the top 20 list, California and New York are dominating the House map – those two states accounted for nearly a quarter of all ad spending in House races in September (24%).

Counting all US House races, California saw more than $30 million worth of ad spending in the month, while New York saw $28.5 million. Pennsylvania ($20 million), Michigan ($17 million) and Arizona ($13 million) were also bombarded with ads.

Across the country, the parties combined for a total of about $245 million spent on advertising for House races in September – including all advertisers, campaigns and outside groups, from both parties – and Democrats led Republicans by about $153 million to $91 million.