As Republicans line up top challengers early in the cycle, Senate Democrats proved with strong first-quarter fundraising that they are not waiting to kick their re-election bids into high gear.

Democratic Senatorial Campaign Committee Chairwoman Patty Murray (Wash.) touted their numbers last week in a briefing with reporters, adding that she began urging incumbents late last year to ramp up their campaigns.

“Every one of them is doing just as we have told them,” Murray said. “Our incumbents raised a total of about $20 million last quarter. In fact, nearly all of our incumbents raised more than $1 million.”

That includes incumbents at the top of the National Republican Senatorial Committee’s target list. First-term Sens. Claire McCaskill of Missouri ($1.1 million), Jon Tester of Montana ($1.2 million) and Sherrod Brown of Ohio ($1.3 million) all recorded strong first-quarter receipts.

Some veteran Democrats in competitive territory had strong quarters as well. Sen. Ben Nelson (Neb.) raised just more than $1 million and has $2.3 million in the bank; Sen. Bill Nelson (Fla.) raised $1.8 million and has $4.6 million on hand; and Sen. Debbie Stabenow (Mich.) took in $1.2 million and has $3 million on hand.

Wisconsin Sen. Herb Kohl’s re-election prospects remain a mystery, as the wealthy basketball team owner raised just $714. However, after loaning his campaign $1 million before the end of last year, he has $954,000 on hand. Democrats say they do not expect any more retirements.

Both Nelsons are already feeling heat from GOP challengers. Nebraska Attorney General Jon Bruning had receipts of $1.5 million and has $1.2 million on hand. In Florida, state Senate President Mike Haridopolos raised $2.6 million and has $2.5 million in the bank.

Tester raised twice as much as Rep. Denny Rehberg (R-Mont.) and holds a $523,000 cash-on-hand advantage. Sen. Bob Casey (D-Pa.) also had a strong quarter, raising $1.1 million and ending up with $2.1 million in the bank. But unlike Tester, he does not yet have a top-tier challenger in the race.

“When I took this job,” Murray said, “one of the things I told our caucus is: We have 23 seats up. We have 23 seats that we have to work hard on, and I expect every one of those candidates to get their campaigns together, to work hard and to do what they need to do to be ready.”

Of course, as Democrats learned last cycle, strong early fundraising does not necessarily portend re-election. Former Arkansas Sen. Blanche Lincoln raised $1.7 million in the first three months of 2009 — long before she barely held on in the Democratic primary and lost by 21 points in the general.

Former Pennsylvania Sen. Arlen Specter raised $1.3 million that quarter, just before switching from the Republican Party to the Democrats. He later lost in the Democratic primary.

Former Wisconsin Sen. Russ Feingold never raised more than $1 million in any quarter of 2009, but he ended up spending a healthy $15 million by the end of the cycle. However, what led to his defeat was a hostile political environment mixed with a Republican challenger who self-funded about $9 million and matched Feingold’s spending.

Even if every Democratic incumbent wins in November, there are enough open Democratic seats for the GOP to win the majority.

Democratic Senators have announced their retirements in Hawaii, North Dakota, New Mexico and Virginia, while Sen. Joe Lieberman (I-Conn.), who caucuses with Democrats, is also retiring at the end of this term. Depending on which party wins the White House, Republicans will need to pick up three or four seats for a Senate majority.

The GOP has a few seats of its own to defend. The most vulnerable include the open Nevada seat, where Reps. Dean Heller (R) and Shelley Berkley (D) are on a collision course for the general election, and Massachusetts, where Sen. Scott Brown is stocking his coffers for a race with an undetermined Democratic challenger.

Brown raised $1.7 million in the quarter, and potential Democratic candidates will have to contend with his Senate-best $8.3 million campaign war chest.

Heller raised roughly the same amount as Berkley in the first quarter, $671,000 to Berkley’s $695,000. Berkley had almost $1.6 million on hand at the end of March, while Heller had $1.4 million.