The Internal Revenue Service headquarters in Washington, D.C. (Susan Walsh/AP)

These are worrisome times for some of the biggest players in Washington’s influence game.

When the IRS announced that it intended to put new limits on the political activities of some tax-exempt groups, it made clear that it was talking about “social welfare” organizations, a category of the tax code that includes several serious spenders in recent elections, such as Crossroads GPS, Americans for Prosperity and the League of Conservation Voters.

But — in what came as a surprise to many — the proposed rules noted that regulators are also considering similar changes that would apply to trade associations and labor unions.

The proposed changes, unveiled in late November, came after calls for greater clarity about the IRS rules on politically active nonprofits following revelations that groups whose names included words such as “tea party” and “patriot” had faced extra scrutiny from the tax agency. But they left many questions unanswered, many details unresolved and many groups unhappy.

And while the changes aren’t expected to be finalized until sometime after the 2014 elections, they’ve become a growing source of concern for a whole host of groups whose bread-and-butter activities could be reclassified as political. The IRS hasn’t yet explained how it would quantify political work or how much would be too much, but groups that cross that threshold would risk losing their status as tax-exempt organizations.

As Brett Kappel, a campaign finance lawyer who advises companies and trade associations, has been telling his clients: “You should pay attention to this. It’s potentially catastrophic.”

The American Society of Association Executives — yes, there’s a trade association for leaders of associations — is also worried.

“If you do start to extend those types of restrictions to trade associations and unions, we believe that would constitute a pretty major shift in what the government believes to be a permissible activity,” said Chris Vest, the group’s public policy director.

Perhaps most significantly, the IRS proposal would define things like get-out-the-vote efforts, voter-registration drives, voter guides and other mentions of candidates — including incumbents, near Election Day — as “candidate-related political activity,” even if the efforts are nonpartisan and don’t tell people how they should vote. If “social welfare” groups did too much of that work, they wouldn’t be able to keep their tax-exempt status.

But trade associations and unions do a lot of that work, too. Just think of the glossy mailings and “legislative updates” that flood your mailbox and inbox, especially during high political seasons.

“The rule is so broadly phrased and so categorical, it would be totally inappropriate for unions,” said John Sullivan, associate general counsel at the Service Employees International Union. “It would seriously affect their ability to function as membership organizations.”

Steve Caldeira, president and chief executive of the International Franchise Association, said by e-mail that his group, too, is watching closely “to ensure that IFA can continue to directly communicate with our members about pro-business, pro-franchising legislators and candidates who support our public policy agenda to improve the business environment for franchising.”

Several groups also complained about the proposal to treat messages to members and other materials as candidate-related activity within 60 days of a general election or 30 days of a primary, but to not consider them political activity outside that window — an especially difficult approach once items have been posted online.

At last count, a whopping 23,005 (mostly negative) public comments about the proposal had been filed, with thousands more likely before the Feb. 27 deadline. They’re mostly focused on what the changes would mean for social welfare groups — created under Section 501 (c)(4) of the tax code — and many use similar language, suggesting that the groups have been encouraging their supporters to weigh in.

Several of the groups from across the political and ideological spectrum have also filed comments themselves. On Tuesday, the American Civil Liberties Union said that it agreed with the effort to define political activity more clearly but that it has “serious concerns . . . both from a First Amendment perspective and as a simple matter of workability.”

The Heritage Foundation, worried about the impact on its advocacy arm, Heritage Action for America, argued that the Treasury Department doesn’t have the authority to impose the rules. And on Wednesday, Cause of Action, a conservative watchdog group, filed suit to delay the rules, calling them “a back door attempt to stifle political opponents” and arguing that they would impose “voluminous recordkeeping requirements on small non-profits.”

Regulating “social welfare” groups is tricky territory for the IRS, which continues to face sharp criticism over its extra scrutiny of politically active groups on the right. And lawyers in the field say they understand the impulse to treat unions, which fall under section 501(c)(5), and trade associations, under section 501(c)(6), like other 501 (c) groups.

But as the SEIU’s Sullivan put it, the current proposal “doesn’t recognize that the organizations are very different in terms of why they were formed and the role they play.”