Republicans aggressively fighting to gain control of the Virginia Senate next month have bankrolled GOP challengers in key races as they try to take the last bastion of Democratic power in Richmond.

The state’s top Republicans, including Gov. Robert F. McDonnell and Attorney General Ken Cuccinelli II, and GOP political committees have focused on five candidates: Bryce Reeves in the Fredericksburg area; Mickey Chohany in Hampton Roads; Sen. William M. Stanley Jr. (Franklin) in Southside; and Adam Light and Del. David A. Nutter (Montgomery), both in southwest Virginia.

In Northern Virginia, they have pumped money into swing or even Democratic-leaning districts, including those held by Sens. George L. Barker (D-Fairfax), Linda T. “Toddy” Puller (D-Fairfax) and Charles J. Colgan (D-Prince William) and an open seat in Arlington County.

Republicans enjoy a huge advantage over Democrats in fundraising as of Sept. 30, according to an analysis by the Virginia Public Access Project, a nonpartisan tracker of money in politics. Republican non-candidate committees have $4.6 million to Democratic committees’ $2.1 million.

The Virginia Republican Senate Caucus took in nearly $370,000 in cash and in-kind contributions for the three-month period that ended Sept. 30 and spent $983,520. More than $630,000 of that money went to the state party. The caucus had $471,0000 in the bank at the end of the reporting period.

The Virginia Democratic Senate Caucus received nearly $2.1 million and spent about $1.3 million. It gave about $720,000 to the state party, which was spent mostly on marketing, research and communications services. It had about $1.6 million in the bank.

Democrats hold a 22 to 18 majority in the Senate. If the GOP picks up two seats, it would seize control because Lt. Gov. Bill Bolling (R) presides over the body.

Republicans gave Stanley nearly $265,000 in the period ending Sept. 30. Stanley announced in May that he would move and seek reelection in the newly redrawn 20th district held by Sen. William Roscoe Reynolds (D-Martinsville). It is the first time in two decades that sitting Virginia senators are pitted against each other in a general election.

“This is a very important seat,’’ Stanley said. “The challenge down here is to unseat a 14-year incumbent.”

Republicans gave $100,000 to Nutter, who is trying to unseat Sen. John S. Edwards (D-Roanoke); $55,500 to Reeves, who is challenging Sen. R. Edward Houck (D-Spotsylvania); $53,150 to Chohany, who faces Sen. John C. Miller (D-Newport News); and $51,500 to Light, who faces Sen. Phillip P. Puckett (D-Russell).

They gave $12,000 to Jeff Frederick, who is running against Puller; $3,500 to Miller Baker, who is challenging Barker; $2,000 to Tom Gordy, who is taking on Colgan; and $26,000 to Caren Merrick, who faces Democrat Barbara Favola in the race to replace retiring senator Mary Margaret Whipple (D-Arlington).

Republicans have put far less into races in the two new districts created during redistricting — in the outer suburbs of Northern Virginia and west of Richmond — signaling they might be more confident about those contests.

The Democratic caucus is mounting a defense, giving big chunks of cash to some of the same races. The caucus gave $70,000 to Reynolds; $65,500 to Houck; $50,000 to Miller; and $55,000 to Edwards.

“They’ve been good to help fill in some gaps,” Houck said. “It looks like they, Republicans, have certainly targeted a small number of us, those of us who represent sort of Republican-leaning districts, I guess. So our caucus, I would assume, is trying to help out in those same races just to keep everything as level as we can possibly keep it.”

The Democratic Party made a few small donations directly to Northern Virginia Senate candidates: $1,000 to Favola, $500 to Puller and $100 to Sen. Janet D. Howell (Fairfax).