(Bloomberg) -- Democrats have ramped up ad buys in several close House races, outspending Republicans by about $500,000 across 31 key House districts six weeks before the general election, according to AdImpact, which tracks political advertising.

In 13 of 31 districts rated as toss-ups by the Cook Political Report, Democrats have now booked more air time than GOP candidates and their allies. That leaves Republicans with more ad spending in 14 districts, down from 18 the previous week. There are four races where ad bookings are virtually tied. 

The ad bookings, which cover the period from the Sept. 5 Labor Day holiday, provide a snapshot of party priorities and which competitive races they think are winnable.  Overall, Democrats have booked $157.1 million of ads compared to $156.6 million for Republicans in the 31 toss-ups.

Republicans need to flip just six seats in November to take the House majority. Democrats are defending 22 toss-up districts compared to nine for Republicans.

The House Majority PAC, a Democratic super PAC with ties to House Speaker Nancy Pelosi, is spending $2.4 million in Arizona’s 1st Congressional District, where incumbent Republican David Schweikert faces a challenge from Jevin Hodge, who heads a local Head Start center. Schweikert, who faced an expensive primary battle, hasn’t booked any air time. 

Democrats also got a boost when the National Republican Congressional Committee, which backs GOP House candidates, pulled its support for J.R. Majewski after the Associated Press reported that he lied about his military record. Majewski, who was endorsed by former President Donald Trump, is running against Democratic incumbent Marcy Kaptur in Ohio’s 9th Congressional District. Democrats are spending $2 million, with the House Majority PAC chipping in $1.3 million of that amount, compared to just $41,000 by Republicans. 

Republicans are spending more in seven toss-ups with incumbent Democrats, and enjoy another advantage. They’re spending more in six districts currently held by Democrats that are now rated lean or likely Republican by Cook. The GOP has booked $33 million in those districts compared to $22 million for Democrats.

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