The new rules would not prohibit political activity by nonprofits.

But by seeking to establish clearer limits for campaign-related spending by groups claiming tax exemption, the I.R.S. proposal could have an enormous impact on some of the biggest groups, forcing them to either limit their election spending or register as openly political organizations, such as super PACs.

A spokesman for Crossroads declined to comment, as did officials at other political nonprofits.

Nick Ryan, the founder of the American Future Fund, which spent at least $25 million on political advertising last year, said, “Unfortunately, it appears that the same bureaucrats that attempted to suppress the speech of conservative groups in recent years has now put together new rules that apply to (c)4 groups but do not apply to liberal groups like labor unions.”

“I wish I could say I am surprised,” Mr. Ryan added, “but I am not.” The final rules are unlikely to be issued until after the 2014 election, after a public comment period.

The administration’s proposal would apply to nonprofits organized under Section 501(c)4 of the tax code, which are granted tax exemption in exchange for devoting themselves to the promotion of “social welfare.”

Under current rules, promoting social welfare can include some political activity, along with unlimited amounts of lobbying. Some of the largest political nonprofits — like Americans for Prosperity, backed by the conservative philanthropists Charles and David Koch — have used that provision to justify significant expenditures on political ads.

But under the new proposal, a broad swath of political work would be classified as “candidate-related political activity” and explicitly excluded from the agency’s definition of social welfare. Those activities include advertisements that mention a candidate within 60 days of an election as well as grants to other organizations making candidate-related expenditures.

“Depending on the details, this could be dramatic,” said Marcus S. Owens, a former chief of the I.R.S.’s exempt organizations division.