When Scott Brown is sworn in today as the 41st Republican Senator, it will be the exclamation point on the Massachusetts GOP’s euphoria that has reigned since his special election upset two weeks ago.

But while Republican candidates have been coming out of the woodwork to launch campaigns against the Bay State’s entrenched Democratic House Members, party officials in Washington, D.C., are taking a more judicious, wait-and-see approach to the idea that Brown’s success could have a trickle-down effect this November.

One national party aide acknowledged that the state is “a little off our radar still,” noting the huge playing field that Republicans are looking at in 2010. Still, the aide said that could change given how quickly things are moving in the wake of Brown’s win.

While Brown won a number of the Democratic-held Congressional districts handily in his Jan. 19 defeat of state Attorney General Martha Coakley (D), Republicans’ House targets in the state will be limited. The most likely: Rep. Niki Tsongas’ (D) 5th district.

Brown topped Coakley in Tsongas’ northeastern district by nearly 30,000 votes and won Tsongas’ home base of Lowell. Tsongas is also the least entrenched Member of the delegation, having won her seat in a closer-than-expected 2007 special election.

Her fundraising is not particularly imposing, either — she reported a total of $581,000 in total receipts in 2009, including $141,000 in the fourth quarter, and ended the year with just $198,000 in the bank.

Republicans are touting challenger Jonathan Golnik, a former currency trader and small-business man, who has impressed state and national officials. Golnik, who met with the National Republican Congressional Committee in Washington just days after Brown’s win, is being advised by members of retired Air Force officer Jim Ogonowski’s 2007 campaign team. Ogonowski came within 6 points of upsetting Tsongas in the special election.

“You look at 2007, Ogonowski lost by just a few percentage points,” said Rob Willington, an adviser to Golnik who worked with Ogonowski as well as Brown. “And that was at the height of a horrible period for the Republican Party.”

But Golnik hasn’t raised much cash — he reported just $51,000 in receipts, including a $5,000 personal loan, since opening a campaign account in mid-October. Ethan Zorfas, a former NRCC aide who oversaw Ogonowski’s finance operation, is directing Golnik’s fundraising. Willington said Golnik’s cash flow has picked up since Brown’s win.

Sam Meas, an investor from Haverhill, is also running in the 5th district GOP field.

Seven-term Rep. Bill Delahunt’s (D) district, which Brown won by nearly 60,000 votes, is also on the radar. State Republicans, in particular, feel optimistic about the South Shore 10th district seat given the emergence of three serious likely candidates — former state Treasurer Joe Malone, state Sen. Robert Hedlund and state Rep. Jeffrey Perry — since Brown’s victory. Malone met with the NRCC on Tuesday, and officials came away impressed.

Massachusetts Republican Party Chairwoman Jennifer Nassour also cited Malone and Perry as examples “of the quality of candidates we’ve had coming out” in the wake of Brown’s upset. In total, the party counts 50 new candidates for state and federal office who have decided to run after Brown’s win. Prior to the special election, 75 candidates had filed.

The NRCC has also been inundated with contacts from newly inspired House candidates.

Like Tsongas, Delahunt has not been aggressive about fundraising. He raised just $58,000 in the fourth quarter, though he still ended the year with $569,000 in cash on hand.

Democrats cautioned against putting too much stock in incumbents’ year-end fundraising numbers, conceding that the delegation had been caught off guard by Brown’s come-from-behind victory and the statewide GOP energy that it fomented. But the element of surprise that benefited Brown is gone. Massachusetts’ Democratic Members are now on full alert, one national party aide said, and next quarter will be a much stronger indicator of how the races stand.

Both sides acknowledge money will be a big factor in how competitive the state’s House races become, given the fact that most of the districts that Republicans hope to put in play are covered by the expensive Boston media market.

It will be nearly impossible for GOP challengers to match Brown’s stratospheric fundraising success in the final weeks of the special election — much of it coming in online from donors around the country galvanized by the chance to capture the late Sen. Edward Kennedy’s (D) seat.

“They’re not going to raise $10 million a week,” Massachusetts Republican National Committeeman Ron Kaufman said of the party’s Congressional hopefuls. Kaufman conceded that the GOP’s candidates will “never be able to match Democrats,” but he didn’t think that would be determinative. “This is a different kind of cycle,” Kaufman said. “I think earned media this cycle is going to mean more than ever before. Voters are paying attention.”

That’s the tack the NRCC is encouraging — earned media, grass-roots outreach and other things that candidates can do on the cheap now to capitalize on the excitement in the electorate and the state party’s vastly expanded voter contact lists and infrastructure.

Kaufman acknowledged that the national party has a lot on its plate this election year, and candidates in the state can’t expect its support in House races. “I kind of feel bad for the Washington party a little bit right now,” he said, adding there is “not any way the party nationally can raise enough money to help all the candidates” running in 2010.

That is especially true given the NRCC’s current cash woes — it started the year with just $2.7 million on hand, six times less than the Democratic Congressional Campaign Committee.

However, Nassour said the state party’s infrastructure is in strong shape, thanks in part to the Senate special election outcome. The expanded crop of candidates, a surge of grass-roots activism, an expanded volunteer corps, a backlash against one-party Democratic rule in the statehouse and Washington, and a strong Republican gubernatorial candidate on the top of the ticket against unpopular Gov. Deval Patrick (D) all make for the best political environment for Massachusetts Republicans in decades.

Added Kaufman, “As we learned with Scott, you’re not going to win a race in Massachusetts with Washington’s help, and we’ve proven that’s okay.”