With the 2006 Senate playing field all but set, the third-quarter fundraising reports filed this week offered the best glimpse yet of where both parties stand in their efforts to build momentum and gain seats next year.

The reports were especially revealing about the dynamics in several key primaries.

Among the top fundraisers for the quarter there were few surprises, as the most vulnerable Senators — and a few 2008 White House hopefuls — continued to pad their campaign coffers.

Sen. Hillary Rodham Clinton (D-N.Y.) again led the way in receipts and available cash at the end of the period, taking in $5.2 million and closing out September with $13.8 million in the bank.

While the possible presidential candidate’s ability to rake in contributions was undeterred, fundraising fell off considerably in the past quarter across the board for her colleagues and Senate candidates. Many campaigns suspended fundraising or canceled fundraisers in the aftermath of Hurricane Katrina, likely one explanation for the noticeable difference from the amount of cash collected during the second quarter of 2005.

Behind Clinton, one Senator and one challenger, both Democrats, topped $2 million raised for the period. By comparison, five Senators raised at least that amount in the second quarter of the year.

Sen. Bill Nelson (D-Fla.) crossed the $2 million threshold for the second straight quarter, the only incumbent other than Clinton to do so.

Nelson is expected to face Rep. Katherine Harris (R-Fla.) next year, although questions about the viability of the second-term Congresswoman’s campaign continue to persist in light of her less-than-stellar fundraising. Rep. Mark Foley (R-Fla.), who is sitting on more than $2.3 million in his campaign account, is still mulling entering the race.

Harris raised $503,000 in the period but spent $438,000, leaving her with just $470,000 in the bank. Nelson, on the other hand, boasted a $6.5 million war chest — the fifth largest among the Senators up in 2006.

Meanwhile in Pennsylvania, pegged early on as the blockbuster Senate contest of the cycle, Sen. Rick Santorum’s (R-Pa.) suffered the most dramatic financial downturn of any incumbent for the quarter.

Santorum raised $1.8 million in the period, less than half of the almost $3.8 million he raised from April through June, and he was one of only two incumbents to be outraised by a challenger.

State Treasurer Bob Casey Jr. (D), who continues to lead Santorum by double digits in most public polls, raked in a whopping $2.2 million in the quarter. Casey’s total was boosted by several high-profile events, headlined by the likes of Sen. Barack Obama (D-Ill.) and Sen. John Kerry (D-Mass.) and his wife, Teresa Heinz Kerry.

Still, the third quarter reports were perhaps most illustrative about the current state of play in several of the primaries on tap for both parties next year.

While both Democratic and Republican leaders are publicly remaining neutral in most of the contests that will decide their nominees in a number of open- seat and challenger races, several of the candidates considered party favorites came out on top financially at the close of the quarter.

In Maryland, Rep. Benjamin Cardin continued to lead the growing field of Democrats vying to succeed retiring Sen. Paul Sarbanes (D). Cardin raised $839,000 in the third quarter — the most of any open-seat candidate for the period. He ended last month with $1.5 million on hand.

Still, it may not be long before one or more of Cardin’s Democratic opponents catches up. Two new entrants into the race, American University history professor Allan Lichtman (D) and forensic psychiatrist Lise Van Susteren (D), both padded their coffers early with personal funds.

Lichtman reported raising $263,000, of which $250,000 was from his own pocket, and Van Susteren raised $251,000 — she gave about $100,000 to her campaign.

Meanwhile, former NAACP head Kweisi Mfume (D), considered a top contender in the race, posted disappointing fundraising totals for a second straight quarter. Mfume reported spending more than he took in ($80,000) and showed a paltry $97,000 in his campaign account.

On the Republican side, state and national party leaders have cleared the field for Lt. Gov. Michael Steele, who is raising money for his campaign but has yet to formally enter the race.

Steele raised $418,000 in the quarter and ended September with $350,000 on hand.

In Minnesota’s open-seat contest Rep. Mark Kennedy (R), the GOP’s anointed candidate, also has a clear shot at the nomination and has continued to stockpile cash for a highly competitive general election.

In the past quarter Kennedy raised $804,000, ranking second in total receipts behind Cardin among open- seat contenders. He ended the period with $1.5 million in the bank.

On the Democratic side, former Hennepin County Attorney Amy Klobuchar continues to lead the three-way field in the race for campaign cash. Klobuchar raised $512,000 in the period and had almost $1.4 million left in reserve on Sept. 30.

Child safety advocate Patty Wetterling (D) raised $216,000 and philanthropist Ford Bell (D) took in $134,000 for the quarter.

In Rhode Island, former state Attorney General Sheldon Whitehouse (D) continues to stand apart from his primary opponent, Secretary of State Matt Brown (D), when it comes to finances.

Whitehouse raised $605,000 in the quarter and showed $1.4 million in his campaign account. Brown raised $405,000 in the period and ended September with $597,000 in reserve.

The winner of that primary will face either Sen. Lincoln Chafee (R) or Cranston Mayor Stephen Laffey (R), who are gearing up for a bruising intra-party fight.

Thanks to a $360,000 personal loan, Laffey far outpaced Chafee in fundraising for the quarter. The mayor reported receipts in excess of $743,000 while Chafee raised just $287,000.

While Laffey still lags well behind the Senator in available cash — $593,000 to Chafee’s almost $1.3 million — his demonstrated willingness to spend personal funds could make the difference a moot point down the stretch.

Meanwhile, the Republican primary in Vermont has been much more low-key so far. Businessman Richard Tarrant (R) is well ahead of military veteran Greg Parke (R) in the money chase. Tarrant reported having more than $400,000 in the bank at the end of last month after loaning his campaign $500,000, while Parke reported just $300. Lt. Gov. Brian Dubie (R) has formed an exploratory committee for a Senate run, but did not raise any money before Sept. 30.

But Rep. Bernie Sanders (I-Vt.), the current frontrunner to succeed retiring Sen. Jim Jeffords (I-Vt.), leads all contenders with $1.2 million on hand. He raised $605,000 during the past three months.

In Tennessee, Rep. Harold Ford Jr. also has a wide lead in fundraising in the Democratic primary, although his cash-on-hand total still lags behind that of one of the GOP candidates.

Ford raised $524,000 in the third quarter, besting all four of the other candidates (of either party) in the race.

Former Chattanooga Mayor Bob Corker (R) raised $519,000 and ended the period with close to $3.2 million in the bank.

While Corker continues padding his coffers, former Reps. Ed Bryant (R) and Van Hilleary (R) are duking it out as they fight for sole control of the conservative mantle in the contest.

Bryant and Hilleary sent out dueling news releases this week touting their financial successes. Hilleary’s was headlined: “Hilleary Out Raises Bryant and Has Over $125,000 More Cash on Hand.” Bryant shot back with this retort: “Bryant out raises Hilleary in money that can be spent in the Republican primary.”

In Nebraska, three Republicans are now vying to take on Sen. Ben Nelson (D) next year. In his first FEC filing, Ameritrade executive Pete Ricketts (R), the favorite of GOP insiders, reported raising $508,000, eclipsing the totals of his two Republican rivals.

Nelson raised $508,000 and showed close to $2.7 million in the bank.

Democrats, meanwhile, face a competitive primary in Montana, where state Auditor John Morrison and state Senate President Jon Tester are battling for the chance to take on Sen. Conrad Burns (R-Mont.). While both Democrats posted a strong fundraising quarter, Morrison ended September with considerably more cash on hand.

Girding for a tough fight, Burns raised close to $1 million in the period and his war chest showed more than $3 million at the end of last month.

Among the other noteworthy totals for Senate challengers elsewhere in the country, Safeco CEO Mike McGavick (R) posted an impressive $720,000 in receipts in his first report to the FEC. McGavick now appears to have a clear shot at the GOP nod to take on Sen. Maria Cantwell (D-Wash.) next year.

Cantwell raised $1.5 million in the third quarter and showed almost $3.9 million in reserve on Sept. 30.

In Missouri, state Auditor Claire McCaskill (D) raised $681,000 in an abbreviated amount of time and ended last month with $645,000 for her campaign against Sen. Jim Talent (R).

Talent showed more than $4 million on hand at the close of the quarter.

And in Arizona, wealthy real estate developer Jim Pederson (D) collected $721,000 in just two weeks on the campaign trail. But the man he hopes to defeat, Sen. Jon Kyl (R-Ariz.), was sitting on more than $4.2 million.

Matthew Reynolds and Jean Chemnick contributed to this report.