The conventional wisdom holds that Newt Gingrich at the top of the ticket (which isn’t going to happen, but still) would be an absolute disaster for Republicans in down ticket races. And there isn’t any reason to doubt that.

But Dems are hoping that in certain races at least, Mitt Romney could also prove a liability to Republican candidates — if Dem contenders can turn him into the walking embodiment of everything that’s wrong, unfair and out of whack about the GOP candidates’ economic policies and prioities.

Case in point: The Massachusetts Senate race. Elizabeth Warren has unleashed a new attack on Scott Brown over his opposition to the Buffett Rule, and she’s tying Brown to Romney in the process:

Just last week, Scott Brown said in an interview that he thinks Mitt Romney and Warren Buffett should get special tax breaks that are not available to most Americans. I don’t think that’s fair.

Senator Brown opposes what’s called the Buffett Rule, which would get rid of the special tax breaks that allow a billionaire like Warren Buffett to pay taxes at a lower rate than his secretary....

Lately, Senator Brown has been telling the people of Massachusetts that we don’t understand Mitt Romney’s tax bracket. No, we understand. Mitt Romney pays 14 percent of his income in taxes. Many people who work for a living pay 25, 28, 33 percent of their hard-earned dollars. Mitt Romney gets a better deal because he earns his income in a way that has been specially protected in the tax code to help people like him.

As I noted here yesterday, the Senate will vote on the Buffett Rule in one form or another. And GOP Senators are likely to vote en masse against it. By all indications, Scott Brown is set to vote against it, too.

This will come as the GOP is set to nominate someone for the top of the ticket who is worth as much as $250 million and would personally profit to an untold degree off the loophole the Buffett Rule would close. If Dems have their way, they’ll be able to turn Romney into the personification of their entire 2012 message about what’s wrong with the economy and tax system, and all the ways they’re rigged for the rich and against the middle class. Obviously Newt would be a far more glaring liability at the top of the GOP ticket, but Dems are hopeful that Romney can be used in some down ticket races to their advantage, too.

Or, as one Dem operative framed the coming message to me: “Are we going to do what’s right for America? Or are we going to do what’s right for Mitt Romney?”