WASHINGTON — With time running short, both progressive and conservative advocacy groups are raising serious objections to new rules proposed by the Obama administration to rein in political activity by nonprofit organizations that are not required to disclose sources of their funding.

In a rare agreement between Tea Party and liberal activists, organizations across the political spectrum say new regulations drafted by the Internal Revenue Service to curb a surge in political spending and activity by nonprofits are far too broad. They fear that enforcement of the regulations would chill more neutral civic initiatives such as voter registration efforts and candidate forums.

A strong backlash from conservatives was anticipated after the I.R.S. made the recommended changes public in November. Many contend that the Obama administration is out to muzzle them, citing the heightened scrutiny the I.R.S. gave to nonprofit applications from Tea Party-affiliated groups. But groups associated with more liberal causes are also calling on the I.R.S. to substantially rethink or withdraw the proposal, criticizing it as overreaching, impractical and undemocratic.

“These rules are a mess, and nobody thinks they are good,” said John Pomeranz, a lawyer who advises progressive nonprofits on election-related activity and lobbying.