Republican Charlie Summers added $507,000 to his Senate campaign bank account between July 1 and Sept. 30, according to his campaign. That brings his total fundraising to about $746,000 since he entered the Senate race in March.

While the figure is more than double the amount Summers raised through June 30, it’s a fraction of the $2.3 million outside Republican groups have spent so far on his behalf — almost entirely on ads attacking independent Senate candidate Angus King.

And that amount of outside, anti-King money is growing. The Washington Post reported Friday that one of those outside groups, the U.S. Chamber of Commerce, is renewing its assault on King with a third ad that attacks his fiscal record as governor, calling him “a major disappointment.”

The U.S. Chamber, which has endorsed Summers, has already spent $839,000 on two other anti-King ads, according to expenditure reports filed with the Federal Election Commission.

Third-quarter fundraising reports from the Senate campaigns are due Monday.

At the close of the last fundraising period in June, Summers lagged King by more than a three-to-one margin: The former governor had raised $897,000 for his bid through June 30 to Summers’ $239,000. The King campaign’s third-quarter finance report will show whether Summers has started to narrow the gap.

Summers had $189,000 in cash on hand as of Sept. 30, according to the campaign, compared to $119,000 at the end of June. And his campaign said half of all third-quarter contributions were in the amount of $200 or less.

While Summers has benefited from spending by outside groups, outside spending meant to benefit King is starting to catch up.

The Democratic Senatorial Campaign Committee has spent more than $700,000 so far on ads attacking Summers, and the nonprofit group Americans Elect has spent almost $900,000 on television advertising and direct mail on King’s behalf.