This morning Greg flagged a fascinating new ad that the Montana Republican Party is running on behalf of Denny Rehberg, the Republican Congressman who is taking on Democratic incumbent Jon Tester in a key Senate race.

The big news here is that Rehberg is campaigning against the House Republican budget, specifically over its threat to Medicare. Rehberg is one of a relatively small group of House Republicans who opposed the Ryan budget. The big question in House races, of course, is what price all of those Members who voted with Ryan will pay, if any.

The very fact that Rehberg is running against his national party in mildly Republican Montana is interesting enough. But it’s also fascinating that in addition to the House budget (and TARP, which is getting a bit old by now), Rehberg’s other big issue where he highlights breaking from his party is trade. Rehberg brags about voting against CAFTA, the Central American trade accord.

This, even as Mitt Romney is very much running as a free trader; one of his most repeated charges against Barack Obama is that Obama hasn’t negotiated new trade deals (a charge which, as it happens, is entirely false).

Getting back to Ryan, the larger context here is revealing. As I noted last month, of 16 Republican candidates most likely to be become new Senators in 2013, only 2 touted the Ryan budget on their web pages. None of them went as far as Rehberg now has gone in specifically denouncing it, but clearly there’s no eagerness to climb about the Ryan train, either because they think it’s bad policy or, more likely, because it polls badly, even in red states such as Missouri, Arizona, and even Nebraska, where nominee Deb Fischer is silent about it.

What’s particularly interesting about this is that the man at the top of the ticket — Mitt Romney — is fully embracing Ryan and his agenda. Which makes you wonder: Are Romney’s people seeing different numbers in their polling about the Ryan plan than the ones Senate candidates are seeing? Or does Romney feel like the right won’t grant him the maneuvering room to distance himself from the unpopular House GOP? Or is Romney a true believer in Ryan economics?

Whatever the explanation, if these Senate candidates are right, the Ryan budget could be politically toxic this fall. The question is whether this will also hurt Romney in an election that will probably be mostly about Obama’s record.