Fourth-quarter fundraising reports filed this week provided the latest evidence that 2010 is developing into an anti-establishment election year, as several Senators reported seven figures’ worth of campaign expenditures in an effort to blunt the negative aspects of incumbency.

At the top of that list is Senate Majority Leader Harry Reid (D-Nev.), who raised an impressive $2.02 million from October to December, but matched it with his equally eye-popping $2.06 million in campaign spending.

Meanwhile, Sen. Barbara Boxer (D-Calif.) dropped almost $1 million to boost her re-election campaign.

While Reid may well be the most vulnerable incumbent of the cycle, Boxer is also facing a serious battle this fall.

But a few sitting Senators have even more pressing concerns when it comes to ensuring they make it to November.

In Pennsylvania, party-switching Sen. Arlen Specter (D) is in for a tough primary against Rep. Joe Sestak in May. Specter raised $1.16 million and spent just under $1.2 million during the last few months of 2009. Sestak had $5.1 million in the bank as of Dec. 31, while Specter reported nearly $8.7 million.

In Utah, Sen. Bob Bennett (R) came close to spending as much as he took in during the fourth quarter. He raised $400,000 from October to December but spent $395,000. Bennett, who is trying to fend off a handful of GOP challengers, reported less than $800,000 in cash on hand at the end of 2009.

Several vulnerable Senators used the last quarter of 2009 to sock away campaign dollars and put some distance between themselves and their competition in terms of cash on hand.

The Republican side of that group included Sens. Richard Burr (N.C.) and David Vitter (La.).

Burr raised $1.2 million during the fourth quarter and reported $4.3 million in cash on hand as of Dec. 31. Those totals were well ahead of all his Democratic rivals, including much-hyped recruit Cal Cunningham, who raised $320,000 in about a month of fundraising and showed $303,000 in the bank on Dec. 31.

In the Bayou State, Rep. Charlie Melancon (D) raised $600,000 for his Senate bid during the quarter, half the $1.2 million raised by Vitter. Melancon also had more than $2 million in cash on hand as of Dec. 31. Vitter had $4.5 million in the bank.

Sen. Blanche Lincoln (D-Ark.), who has struggled in recent polling on her race, far outraised her top would-be GOP challengers. Lincoln raised $1.3 million during the quarter and showed $5 million in the bank as of Dec. 31. Her most touted Republican opponent, state Sen. Gilbert Baker, raised less than $300,000 and reported $639,000 in cash on hand. Rep. John Boozman (R-Ark.), who is set to join the field this week, ended the year with just $292,000 in the bank.

In Colorado, appointed Sen. Michael Bennet (D) pulled in $1.16 million during the fourth quarter and had $3.5 million in the bank at the end of the year. Despite facing a primary challenge, those numbers still put him on solid financial footing. Ex-Lt. Gov. Jane Norton is the leading Republican in the race.

In two open-seat Senate contests, fourth- quarter fundraising reports confirmed that one-time GOP frontrunners are falling back to earth in their respective primaries.

Nowhere is that more evident than in Florida, where Gov. Charlie Crist (R) faces a primary against former state House Speaker Marco Rubio.

Crist’s $2.1 million in fundraising during the quarter was down from the $2.5 million he raised in the third quarter and $4.3 million he raised in the second quarter of 2009. And as Crist’s totals have decreased, Rubio has watched his fundraising take off.

Rubio posted his best quarter yet during the final three months of 2009, raking in $1.76 million. That’s up from just more than $1 million in the third quarter of 2009.

In Kentucky, Secretary of State Trey Grayson is the GOP establishment’s choice — not to mention the political protege of Minority Leader Mitch McConnell (R-Ky.). Despite that support, Grayson was outraised for the second quarter in a row by anti-tax activist Rand Paul, the son of Texas Rep. Ron Paul (R). Grayson raised $446,000 to Paul’s $653,000. As of Dec. 31, Grayson had $1.4 million in cash on hand to Paul’s $1.3 million.

Daniel Newhauser and Zack Hale contributed to this report.