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WASHINGTON (Reuters) - A leading Republican political action committee that has spent heavily to influence next month’s congressional elections told donors on Tuesday that fewer than two dozen races will determine which party controls the U.S. House of Representatives.

The Congressional Leadership Fund, a super PAC closely linked to retiring House Speaker Paul Ryan, has already spent $90 million on key House races, but warned its donors in a memo dated Tuesday and seen by Reuters that more cash is needed to avert a Democratic takeover of the House.

The memo, from CLF executive director Corry Bliss, said internal polling had shown that voter approval of Republican President Donald Trump had risen by 5 percentage points in 20 critical House districts since the bitter Senate fight over Supreme Court Justice Brett Kavanaugh.

Kavanaugh, who was accused of sexual assault while a teenager, was confirmed by the Senate on Saturday and sat with the court for the first time on Tuesday.

“There are 20 races within 4 points that will determine the House majority, and CLF will keep working to win them,” Bliss wrote in the memo.

Democrats need to gain a net total of 23 seats in the Nov. 6 elections to assume control of the 435-seat House, which would allow them to would derail much of Trump’s agenda and open his administration up to greater oversight. Independent analysts have said widespread discontent with Trump and the Republican Party could lead to Democrats winning as many as 40 seats.

Republicans argue that the controversy over Kavanaugh has engaged conservative voters who otherwise have been apathetic to the midterm elections. Democrats contend the fracas has added to their edge in voter enthusiasm, particularly among women voters.

The memo warns that Democrats have outspent their rivals by $50 million in key races.

On Tuesday, the CLF’s Democratic counterpart, House Majority PAC, said it would spend $20 million in new television advertising in 13 competitive House districts, including six in California that Democrats hope to flip.