One key metric for determining whether the attacks on Mitt Romney and the bruising GOP primary are damaging him for the general election is this: How is he faring among independent voters, a key swing constituency, now that they’re getting to know him better?

I’ve got a new partisan breakdown of some numbers from the new NBC/WSJ poll, and here’s the verdict: Romney’s unfavorability rating among independents has spiked 20 points in the last two months.

The poll found that among overall Americans, Romney is rated very or somewhat positively by 31 percent, while he’s rated very or somewhat negatively by 36 percent.

The office of pollster Peter Hart, who helped do the survey, sends over the numbers among independents. While Romney’s positive numbers among them have been roughly stable, at just over 20 percent, here is the change among independents in the past few months:

In November, Romney was rated somewhat or very negatively by 22 percent of independents.

In December, Romney was rated somewhat or very negatively by 29 percent of independents.

And in the new poll, Romney was rated somewhat or very negatively by 42 percent of independents — 20 points higher than two months ago.

Also: In November, Romney was beating Obama 47-34 among those voters. Now the numbers are upside down: Obama is beating Romney 44-36.

(A recent Post/ABC News poll also found Romney’s negative rating among independent had jumped to 51 percent.)

Pollster Peter Hart, a Democrat, tells me Romney’s multiple gaffes and revelations about his taxes and Bain background have led independents to start making a choice between Romney and Obama, rather than merely looking at Romney as a generic opponent of the President.

“Romney’s lack of connection with average people and Obama’s improving numbers on the economy account for the turnaround with independents,” Hart said.

“It’s not as though they have said Bain has disqualified him or that he can’t be trusted because of his taxes — but this has created a gulf between him and the average voter,” Hart said. “Bain and the taxes just reinforce the sense that this person is in a different world. If you’re an independent voter not driven by partisanship, you read all the other tea leaves to decide who is going to fight your fight.”

“When he was only the opponent of Obama, he was getting the independent vote as the alternative,” Hart says. “Now they see him in full relief, and they’re saying, `This isn’t my guy.’