Wis. Gov. Scott Walker makes a stop at the Republican Party headquarters in Appleton, Wis. (Sharon Cekada/AP)

A group at the center of a campaign finance investigation ensnaring Wisconsin Gov. Scott Walker has an emphatic retort to prosecutors examining whether its coordination with politicians was illegal: Everybody does it.

If Walker broke the law, “perhaps the majority of politicians in Wisconsin and across the nation would be at risk of prosecution and conviction,” attorneys for the Wisconsin Club for Growth wrote in a recent court filing seeking to block the inquiry. To underscore the point, the lawyers cited efforts by some of President Obama’s top aides and administration officials to raise money for an independent super PAC supporting the president’s 2012 reelection bid.

The investigation of Walker (R) and his allies has been challenged as unconstitutional, and it is unclear whether it will proceed.

Still, campaign strategists and legal experts nationwide are closely watching the inquiry as a major test of what practices cross the line in the loosely governed and increasingly murky area of big-money politics.

The growing influence of super PACs and politically active nonprofits, which can raise unlimited money, has eroded the once-thick wall between official campaigns and outside interest groups.

Candidates, still bound by contribution limits, are restricted by state and federal laws from sharing political strategy with outside organizations. But most of the coordination rules are narrowly drawn, with little oversight from federal regulators, giving groups and candidates leeway to push the boundaries.

This has led Democrats and Republicans alike — fearful of losing tactical ground — to step up engagement with well-funded outside allies.

Top consultants, for instance, often shuttle back and forth between a candidate’s campaign and outside groups. Elected officials regularly headline fundraising events for their ostensibly independent allies, as Obama did in New York last week for a Democratic super PAC seeking to influence Senate races.

“They are trying to do as much as they can to cooperate without illegally coordinating — which, in truth, is not that difficult to do, because the line for what counts as coordination is a particularly high bar,” said Daniel P. Tokaji, who teaches election law at Ohio State University and co-wrote a newly released study on outside spending in congressional races.

The Walker case “raises the stakes” for political players who have “gotten undisciplined in maintaining independence,” said campaign finance lawyer Kenneth Gross, a former Federal Election Commission enforcement chief. The investigation could encourage the Justice Department to look at similar episodes on a federal level, he said.

Walker, who faces a competitive reelection campaign this year and is pondering a presidential run in 2016, called the accusations revealed in documents unsealed last week “categorically false” and said the inquiry, begun by a Democratic district attorney, was “nothing more than a partisan investigation with no basis in state law.”

A federal judge shut down the inquiry last month, saying the coordination did not violate election laws because the outside groups were engaged in issue advocacy, not explicit political activity.

The case is now pending before a federal appeals court. Its resolution could bring an unusual moment of clarity to a system in which the rules seem to grow more confusing by the year.

The co-dependency of candidates and their rich allies can be seen in the increasingly creative ways the two sides are working together, albeit indirectly.

Political parties, for instance, are restricted from joining forces with independent groups. Yet this year, the Democratic Senatorial Campaign Committee has tweeted out links to gauzy biographical footage of its candidates, along with suggested scripts attacking their GOP rivals — ingredients for ready-made TV spots to be produced and aired by outside super PACs. The GOP’s House campaign committee has a Web site with customized opposition research on Democrats, sorted by district, clearly designed to assist conservative organizations drafting attack ads.

Campaign finance lawyer Robert Lenhard said he fields weekly queries from clients wondering about the boundaries of coordination. Political operatives are “looking over the fence, trying to see what the other folks are doing, not wanting to be competitively disadvantaged,” said Lenhard, a former FEC commissioner.

It’s rare now for operatives to go to the lengths that former White House aide Bill Burton did in early 2011, when — after starting the pro-Obama super PAC Priorities USA Action — he unfriended former colleagues on social media to avoid the perception of even accidental coordination.

“We basically went two years without talking to a lot of our good friends,” Burton said.

Even so, White House officials appeared at fundraising events for the super PAC — a tactic the FEC approved in 2011. Such interplay between top Obama aides and the super PAC was cited by the Wisconsin Club for Growth in its court filing as an example of how coordination is taking place across the political landscape.

Some advocates for tougher enforcement argue that the FEC is not even applying the rules that are on the books.

The Campaign Legal Center, a nonpartisan group that favors stricter campaign finance regulation, has challenged the growing use by outside groups of “b-roll” footage posted on candidate Web sites for use by allied groups preparing TV ads. Such footage was dubbed “McConnelling” by “The Daily Show” host Jon Stewart, who mocked a video showing a smiling and hardworking Senate Minority Leader Mitch McConnell (R-Ky.) in various settings.

Candidates in both parties have widely adopted the tactic this year, but critics say the practice amounts to an illegal in-kind contribution.

“Some of the activity we’ve seen this cycle is plainly illegal,” said Paul S. Ryan, senior counsel for the Campaign Legal Center.

The Walker case stems from the outpouring in Wisconsin by conservative groups in 2011 and 2012, after the governor became a national hero on the right for his push to roll back the collective-bargaining rights of public-sector unions.

Labor groups staged mass demonstrations and supported efforts to recall Walker and Republican lawmakers in hopes of shifting the balance of power.

Walker survived, as did the GOP’s state Senate majority, in part because of the influx of money from conservative organizations.

Strategists on the right have come to describe the victories there as the “Wisconsin model,” citing the impact that was possible through intensive spending in a single state.

The Walker case is playing out as, once again, groups on both sides are preparing to assert a big role in Wisconsin politics. Walker’s Democratic challenger, Mary Burke, is being supported by state and national labor unions, which have vowed to spend aggressively against the incumbent.

Sean Sullivan contributed to this report.