If there is one thing that’s central to Senator Scott Brown’s reelection hopes, it’s his image as an independent politican who’s willing to work across party lines and doesn’t take his marching orders from the national GOP. Polls suggest he’s been very successful at cultivating that image.

That’s why Democrats were so delighted by what they see as a major strategic blunder on his part: The news that he’s raising money from national Republicans by invoking the menace of an Obama second term.

HuffPo’s Michael McAuliff reported late yesterday that Brown circulated a fundraising appeal describing his Senate race in urgent terms. “This race is THE battleground for the United States Senate — the only sure hedge to a potential second term for President Obama,” Brown’s appeal says.

HuffPo reported that the fundraising letter appeared to have gone to an out of state list — meaning he likely is raising money from a national conservative audience by portraying his race as the last bulwark against creeping Obama tyranny.

I’m told that Democrats are going to incorporate this into their narrative against Brown. Elizabeth Warren’s campaign went out with a fundraising appeal today that spells out the new message:

On TV, Scott Brown says that he’s an independent voice in the Senate.

But that’s not the message he sends to his campaign supporters. In fact, when the cameras aren’t rolling, you see Scott Brown’s true colors — and he’s as partisan a Republican as they come...

Scott tt Brown likes to hide his allegiances when it’s politically convenient, but it’s crystal clear: A vote for Scott Brown is a vote for President Mitt Romney...

Donate $5 to deliver Scott Brown a clear message: “I’m supporting President Obama, and I’m supporting Elizabeth Warren.”

In other words, Dems will use this to appeal directly to the huge bloc of Massachusetts voters who didn't come out in the special election Brown won in 2010, but will come out in a presidential year. Many of them may be Obama supporters who might entertain voting to reelect a likeable GOP incumbent they perceive as independent of the national party.

Brown has repeatedly appeared at White House bill signings and has aggressively promoted his role in championing the STOCK Act banning insider trading in Congress, which Obama signed earlier this month. So Dems have work to do in undermining Brown’s independent, bipartisan image. But this will be pressed into service, alongside Brown’s votes for the Blunt Amendment and against the Buffett Rule, as a key data point against him.