Sen. Benjamin Cardin (D-Md.) announced Friday that his reelection campaign had raised more than $700,000 from July through September, demonstrating again that any challenger will have a steep financial hill to climb.

Cardin, who is running for his second term, had more than $2.3 million in the bank as of Sept. 30. “Particularly during these tough economic times, I am humbled by the level of support I’ve received,” Cardin said in a statement issued by his campaign.

Former Secret Service agent Daniel Bongino, the most active Republican candidate in the race so far, will report a much smaller haul. Bongino raised $52,000 in the 3rd quarter, and had $31,000 on hand at the end of September.

“We’re actually encouraged by it,” said Bongino campaign aide Jim Gibbons, noting that Bongino, a first-time candidate, had no name identification or fundraising infrastructure before the entered the race.

Bongino was endorsed this week by Sen. Mike Lee (R-Utah) and is hoping to boost his fundraising by attracting support from more prominent Republicans soon.

Del. Patrick L. McDonough (Baltimore County) and 2010 senate candidate Eric Wargotz are also mulling bids for the Republican nomination.

On the Democratic side, state Sen. C. Anthony Muse (Prince George’s) has announced the formation of an exploratory committee to mount a primary challenge against Cardin. But Muse has not yet filed papers with the Federal Election Commission.