At least 10 Senators up for re-election in 2006 raised $1 million or more in the second quarter of the year, while seven are brandishing campaign accounts that surpass $4 million, newly filed fundraising reports show.

In a scenario likely to be replicated throughout the remainder of the cycle, first-term Sen. Hillary Rodham Clinton (D-N.Y.) set a scorching fundraising pace for the quarter and continued to far outdistance her 28 colleagues who are also facing voters next year.

Clinton, the leading potential Democrat in the 2008 presidential race, raked in more than $6.1 million in the three-month period and ended June with nearly $12.6 million in the bank.

Sen. Rick Santorum (R-Pa.), regarded as the most vulnerable incumbent up in 2006, had the second-largest fundraising total in the quarter, taking in $3.7 million for his re-election effort. Santorum is likely to face Pennsylvania Treasurer Bob Casey Jr. (D), who raked in close to $1.9 million.

Casey was one of three Senate hopefuls who surpassed the $1 million mark for the quarter. Maryland Rep. Ben Cardin (D) and New Jersey Rep. Bob Menendez (D) raised $1.2 million and $1.9 million, respectively. Cardin is seeking to replace retiring Sen. Paul Sarbanes (D-Md.), while Menendez is among the leading candidates to succeed Sen. Jon Corzine (D-N.J.), if Corzine is elected governor this November.

Rounding out the top five fundraisers among incumbents for the quarter were Sens. Jim Talent (R-Mo.), $2.5 million; George Allen (R-Va.), $2.4 million; and Bill Nelson (D-Fla.), $2.3 million.

Of that group, Nelson is the only one so far who has already drawn a competitive challenge. Democrats are aggressively wooing Missouri Auditor Claire McCaskill (D) to run against Talent, while attacking Allen for putting his 2008 presidential ambitions above his re-election efforts.

Rep. Katherine Harris (R-Fla.), who has announced she will run against Nelson next year, raised $403,000 in the period and showed $407,000 in the bank on June 30.

Among the top seven cash leaders — those who have at least $4 million in their bank accounts — Nelson and Santorum are the only Senators all but assured of a competitive race next year.

Sens. Dianne Feinstein (D-Calif.), Edward Kennedy (D-Mass.) and Kay Bailey Hutchison (R-Texas) are expected to face only nominal opposition, while Clinton and Allen have yet to draw top-tier opponents.

Lawyer Ed Cox (R), son-in-law of the late President Richard Nixon, raised $768,000 for his bid against Clinton but after spending $639,000 in the quarter he ended June with just $129,000 in the bank. He is not expected to put much of his own money into his Senate campaign.

For the second quarter in a row, Sen. Lincoln Chafee (R-R.I.) was the only Senator out-raised by a challenger in the period.

Chafee, a top target for Democrats in 2006, took in $436,000 and ended June with $1.1 million in reserve. Former Rhode Island Attorney General Sheldon Whitehouse (D) raised $783,000 in the quarter and showed just more than $1 million in his campaign war chest at end of last month. Secretary of State Matt Brown (D) is also gunning for Chafee and he raised $303,000 for his Senate bid. However, Brown lags far behind both Chafee and Whitehouse in cash on hand, after spending the most out of the three Senate contenders ($234,000). He ended June with $467,000 in the bank.

At least two other non-incumbent candidates spent large sums in the quarter, relative to the cash they took in.

In Michigan, the Rev. Keith Butler (R) raised $830,000, but spent more than half of it, $426,000, and ended the period with $404,000 on hand. He has lent his campaign roughly $70,000 in personal funds so far.

Butler is challenging Sen. Debbie Stabenow (D-Mich.), who raised $1.4 million in the quarter and ended June with close to $3.9 million in her campaign war chest.

Meanwhile, in Minnesota’s open Senate race, real estate developer Kelly Doran (D) raised $326,000 ($270,000 of which came in the form of a personal loan) and spent $304,000, leaving him with $22,000 in available cash.

Rep. Mark Kennedy (R-Minn.) led all of the contenders for retiring Sen. Mark Dayton’s (D-Minn.) seat in the second quarter money race, bringing in $758,000. Still, Kennedy trails Hennepin County Attorney Amy Klobuchar (D) in available funds for the race. Klobuchar raised $650,000 and showed $1.1 million in cash on hand at the end of June.

Among the other three open-seat races, there were a handful of fundraising standouts.

In Tennessee, former Chattanooga Mayor Bob Corker (R) raised $716,000 and reported $2.9 million in his campaign account. Former Reps. Ed Bryant and Van Hilleary each raised less than half of Corker’s total in the period — taking in slightly more than $300,000 each.

Rep. Harold Ford Jr., the leading contender for the Democratic Senate nomination, raised $695,000 in the quarter and showed close to $1.8 million in his war chest.

Meanwhile, in Maryland, Cardin got off to a brisk start and holds a wide fundraising advantage over former NAACP President Kweisi Mfume (D), who reported raising $135,000 and had $119,000 left in reserves. Cardin showed $1.2 million in the bank on June 30.

Maryland Lt. Gov. Michael Steele, the presumptive Republican nominee, just opened an exploratory committee last month and did not raise enough money in June to file with the FEC.

In Vermont, Rep. Bernie Sanders (I) came just shy of hitting the $1 million mark in funds raised, taking in $973,000 in the period. Greg Parke, the only announced Republican in the race, took in $143,000 and had just $16,000 left in the bank at the close of the quarter.

Teddy Davis and Matt Reynolds contributed to this report.