The first-quarter campaign finance reports filed recently by Senate candidates help explain why Republican strategists are optimistic about picking up a large number of seats this fall.

In several key tossup races, the confirmed or likely Republican nominees topped
$2 million in receipts for the period and easily outpaced their Democratic opponents.

“The Democrats’ unpopular big government agenda is not only helping our Republican candidates in the polls, but it’s helping them stock their campaign war chests as well,” National Republican Senatorial Committee spokesman Brian Walsh said.

One of the biggest partisan imbalances in fundraising is in Ohio, where former Rep. Rob Portman (R) raised about $2.4 million in the quarter — more than four times as much as the $554,000 posted by Lt. Gov. Lee Fisher, the leading Democrat.

Portman has been able to save his money because he doesn’t face a contested primary. But Fisher must get past Ohio Secretary of State Jennifer Brunner (D), who is running a sparsely funded campaign, and he spent nearly as much as he raised in the period. That allowed Portman to extend his cash-on-hand lead to $7.6 million verses Fisher’s $1.8 million as April began.

Republicans are at least even-money bets to capture Democratic-held seats in Illinois and Pennsylvania in part because their candidates are well-funded.

In Illinois, Rep. Mark Kirk (R) outraised state Treasurer Alexi Giannoulias (D) $2.2 million to $1.2 million in the first quarter — a period that included the Feb. 2 primaries that nominated Kirk and Giannoulias. Kirk began the spring with more than twice as much money to spend than Giannoulias, $3 million to $1.2 million.

GOP strategists were also pleased that former Rep. Pat Toomey added $2.3 million to his campaign war chest to unseat Sen. Arlen Specter, who is being challenged in the Democratic primary by Rep. Joe Sestak. Though the cash-on-hand totals reported by Specter ($9.1 million) and Sestak ($5.2 million) are higher than Toomey’s $4.1 million, the Democrats will have to unload significant amounts of those treasuries as they approach the homestretch of their May 18 primary.

The most eye-popping figure of the quarter belonged to Marco Rubio (R), the former Florida Speaker who has galloped past Gov. Charlie Crist to become the frontrunner to win an open seat. Rubio raised almost $3.7 million, more than any other Senate candidate in the nation who didn’t self-finance and more than three times as much as the $1.1 million raised by Crist and Rep. Kendrick Meek (D).

In the open-seat race in Kentucky, Republicans Rand Paul and Trey Grayson significantly outraised Democrats Jack Conway and Daniel Mongiardo.

The two parties essentially fought to a draw in Missouri and New Hampshire, two more states where GOP Senators are retiring.

In Missouri, Secretary of State Robin Carnahan (D) edged Rep. Roy Blunt (R), $1.5 million to $1.3 million, but trails in cash on hand, $3.5 million to $2.8 million.

In New Hampshire, former state Attorney General Kelly Ayotte (R) narrowly outraised Rep. Paul Hodes (D), $672,000 to $665,000, even though Ayotte is running in a contested primary and Hodes will coast to the Democratic nomination. Hodes leads Ayotte in cash on hand, $1.7 million to $1.3 million. And Ayotte will have to spend more than she’d like before a September primary that also includes Bill Binnie, a wealthy businessman who is primarily self-financing his campaign.

The GOP did have a disappointing showing in Indiana, where former Sen. Dan Coats raised just $379,000 in about two months of fundraising — not the kind of showing you might expect from a former lawmaker who later became a K Street lobbyist.

Coats’ modest fundraising start ensures that Rep. Brad Ellsworth (D) will begin the general election campaign with a significant financial advantage. Ellsworth raised $627,000 in the first quarter and began April with about $1.1 million in his coffers.

Several Democratic Senators in the GOP crosshairs posted first-quarter receipts well into the seven figures. Majority Leader Harry Reid (Nev.) took in $1.8 million and had $9.4 million in the bank, more than any Senator in a competitive race. Sen. Barbara Boxer (Calif.) took in about $2.5 million and began April with $8.7 million as she awaits the winner of a contested Republican primary.

“Our candidates continue to grow their campaigns, and supporters are energized more than ever by Republican obstructionism,” Democratic Senatorial Campaign Committee spokesman Eric Schultz said.

Two Republicans who are being seriously challenged in their own party, Sens. John McCain (Ariz.) and Bob Bennett (Utah), ramped up their fundraising in the first quarter.