In the race to succeed Edward J. Markey in the US House of Representatives, Middlesex Sheriff Peter J. Koutoujian’s campaign raised more than $600,000 in about three months, a sign of strength as the Oct. 15 special Democratic primary election looms.

From July 1 to Sept. 25, the Waltham Democrat’s campaign raised $610,000, according to filings with the Federal Election Commission, and ended that period with $690,000 in the bank.

The amount of money a candidate raises is a key and very public indication of the campaign’s health, especially so close a decisive election.

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Koutoujian and six other Democrats are vying to represent the heavily Democratic district, which lost its Congressman when Markey became a US Senator.

Among the other contenders:

•State Senator Katherine Clark’s campaign pulled in $616,000 over the almost three-month time period, according to campaign manager Brooke Scannell. But that total included a $250,000 loan from the candidate, according to her federal filings. Clark’s effort had $386,000 in the bank on Sept. 25, Scannell said.

•State Representative Carl M. Sciortino’s campaign raised $266,000during the July 1 to Sept. 25 reporting period mandated by the Federal Election Commission, and had $285,000 in cash on hand on Sept. 25, according to filings.

•State Senator Will Brownsberger’s campaign raised $215,000 during that period and had $239,000in the bank on Sept. 25, according to a campaign aide.

•State Senator Karen Spilka’s campaign raised $208,000 and had only $132,000 in cash on hand on Sept. 25, according to filings.

Also running in the Democratic primary are Paul John Maisano, who works in the construction industry, and Martin Long, an author. Neither of their reports were available on the Commission website early Friday.

The three Republicans battling for their party’s nomination in the Fifth District: actuary Tom Tierney; Harvard nanophysics researcher Mike Stopa; and businessman and lawyer Frank J. Addivinola Jr.

The special general election will be held Dec. 10.