American Crossroads, the largest of a new generation of political interest groups, is seeking to use a loophole in campaign laws to do something that has occupied a gray area of election law: produce advertisements featuring federal candidates and officeholders.

That would be a step that Crossroads and other “super PACs” have not taken so far. But the conservative Crossroads says it is just following a precedent set by the Democratic Party, which has spent at least a half-million dollars in recent months on ads featuring Sen. Ben Nelson (D-Neb.) talking about his record.

The ads backing Nelson blow past the legal cap on how much money the party can spend to help a senator’s campaign, but party officials say the ads don’t run afoul of the law.

The trick is an old one: Run ads that are not considered election ads under the federal standard and you can get around the tighter restrictions on campaigning. The spots are dubbed “issue ads” because they are nominally about issues instead of elections. To the average viewer, the difference is imperceptible.

The Nebraska Republican Party has filed complaints with the Federal Election Commission and the Senate ethics committee over Nelson’s appearance in the ads, saying it violates election rules.

“Perhaps Senator Nelson will attempt to explain these ads away as something other than campaign ads, and add to his unfortunate record of telling Nebraskans one thing while doing the opposite,” the Nebraska GOP said in its FEC complaint.

In the ads, Nelson touts his vote against increasing the debt ceiling, saying the budget deal didn’t cut enough federal spending and dipped too far into Medicare. “Like most Nebraskans, I can smell a skunk, and that deal stunk even for Washington,” Nelson says in the ad.

Democrats say they’ve done the same thing before with little uproar, both in 2006 to help Nelson’s reelection and in 2008 to help Jeff Merkley in his successful Senate bid in Oregon.

Oregon Republicans filed a similar complaint against Democrats in the state over the Merkley ads, but the FEC’s general counsel suggested that the ads were legitimate, and the commission voted not to investigate the matter.

“The Republican Party is trying to bring out the stink pot and turn this into something to their advantage,” said Paul Johnson, Nelson’s campaign manager.

Nelson has not announced whether he plans to run again. Republicans are eyeing his seat, and it will probably be one of a handful that decide whether Democrats keep control of the Senate.

As one of the most conservative Democratic senators, Nelson has emerged as a key roadblock to President Obama’s agenda, making the party’s decision to support him with a substantial advertising buy this far ahead of the election all the more remarkable. On Tuesday, Nelson was one of only two Democrats to oppose Obama’s jobs bill in a Senate vote.

“Nebraska Democrats don’t expect a lap dog,” Johnson said. “One of the things that we admire is his independence.”

Currently, Crossroads and other interest groups are not allowed to coordinate with candidates on election spending. (The political parties can coordinate with candidates, but only up to a certain spending limit.) On Wednesday, Crossroads filed a request with the FEC asking whether it could produce advertisements featuring candidates, using the Democrats’ rationale.

The FEC is unlikely to approve Crossroads’ request outright, but there’s a good chance the commission’s Democratic and Republican members would split 3 to 3 on the matter. Many interest groups would take that as a green light, because it takes four members of the commission to open an investigation or impose fines.

If Crossroads and other super PACs were to use candidates in advertising, it would further complicate the patchwork of election laws designed to restrain donations to politicians.

By law, such interest groups are prohibited from coordinating on elections with federal candidates, officeholders and even party officials. The Supreme Court has found these rules are sufficient to isolate politicians from the corrupting influence of large donations and spending.

That wall between politicians and big money took a hit when the FEC found that candidates could help raise money for independent groups such as super PACs as long as they asked donors only for checks within the $5,000 limit allowed for regular political action committees.

Republican presidential candidate Mitt Romney and Democratic lawmakers have already begun soliciting contributions for the independent groups supporting them. If candidates like them are able to help create ads as well, it would take independent groups one step closer to an official campaign operation.