On Wednesday night, Indiana Sen. Richard Lugar joined the lion’s share of his Republican colleagues in voting for a GOP budget authored by Wisconsin Rep. Paul Ryan that some say may cost them in the 2012 election.

Lugar, one would think, had little choice. He faces a spirited primary from the right in 2012, from Indiana state Treasurer Richard Mourdock. And proving his conservative bona fides is paramount.

The problem is, while Mourdock says he would have supported the Ryan budget, even he is treading lightly around the budget’s Medicare provisions, which Democrats say is boxing Republicans in politically.

“[Mourdock] would vote ‘yes’ for the Ryan budget, although he still has serious misgivings regarding long-term budget implications,” spokesman Chris Conner said when asked for comment by The Fix. “He believes it is a reasonable first start.”

Lugar was more gung-ho about voting for the bill, announcing his support days before the vote.

That role reversal — the supposedly moderate senator embracing a controversial conservative budget vision while his conservative opponent more reluctantly embraces it — is perhaps the best example of how the issue is beguiling Republicans across the country.

Most Republican candidates are smartly declining to take a firm stand on the issue, which is a tacit acknowledgement of just how dicey it is for them politically.

And relatively few are using it to try and stake out conservative ground in competitive primaries.

Two GOP senators potentially vulnerable to primary challenges — Sens. Scott Brown (Mass.) and Olympia Snowe (Maine) — nonetheless voted with Democrats against the budget, while Utah Sen. Orrin Hatch, who faces a potential party convention challenge next year, voted with the majority of his GOP colleagues.

In Montana, Rep. Denny Rehberg (R) was one of just four House Republicans to vote against the bill last month. Rehberg is unlikely to face much of a primary challenge in his bid against Sen. Jon Tester (D-Mont.), though he may have invited one with the vote.

“I think that says a lot about how potent they know this (issue) is,” said Democratic Senatorial Campaign Committee Chairman Patty Murray of the ‘no’ votes from Rehberg, Snowe and Brown.

GOP Senate candidates like Mourdock have been rather silent on the issue, though some began taking positions when the Senate voted on the budget earlier this week.

Among those Senate candidates coming out strongly in favor of the Ryan budget were Dallas Mayor Tom Leppert, whose conservatism has been questioned by some in the state and former Florida state Rep. Adam Hasner, who was about the only major Senate candidate to try to use the issue to create a contrast with his Republican opponents.

“Unlike my opponents, I would vote for the plan without hesitation, because I know that the alternatives are rationed care and declining health care options, watching Social Security and Medicare slowly go bankrupt, or America faltering under the weight of unsustainable entitlement programs,” said Hasner, who even suggested the plan “does not go far enough.”

But Hasner’s maneuvering is the exception rather than the rule.

Many other Senate candidates, including former Rep. Heather Wilson (R-N.M.) and former senator George Allen (R-Va.) have been reticent to embrace Ryan’s budget. (For a more detailed rundown of candidates’ positions, check out Sean Sullivan’s piece over at the Hotline.)

Even the most conservative Republicans seem to be shying away from it, knowing that they risk losing support from both independents and potentially even elderly Republicans due to the Medicare provisions.

This, of course, is the smart course politically — candidates generally are advised not to take positions on tough issues unless they have to — but it only works if there’s not a lot of pressure on you to make a decision.

Case in point: former Minnesota governor Tim Pawlenty was essentially baited into taking a yes-or-no position on the Ryan budget Thursday after a number of news outlets reported that the presidential candidate had declined to do so. (Senate candidates, of course, can fly under the radar more than presidential ones can.)

For now, the issue has died down a bit. But if House Republicans and the party’s activist base continue to push the envelope, it will be harder for Senate candidates to keep quiet. And that will put them in a very uncomfortable political position.