Other groups that register and mobilize voters may be vulnerable, too. America Votes, a group with a permanent staff in more than 20 states that turns out voters on Election Day and rallies them around causes like reproductive rights and the environment, received more than one-fifth of its nearly $10 million budget from the four biggest public-sector unions in 2016.

The major public-sector unions are also major backers of ballot-measure campaigns, having spent more than $7 million on such efforts in 2015 and 2016. Some of the measures involved increasing taxes to fund services like education and health care, and raising the minimum wage. Unions funded opposition to a measure allowing an expansion of charter schools in Massachusetts.

(Some unions said it was too early to know what spending they would cut. The American Federation of Teachers was adamant that it would continue funding certain groups, like America Votes.)

In some arenas, the loss of union money will almost certainly be offset by other liberal donors — both the wealthy and ordinary citizens. Few party operatives worry, for example, that the Democrats will lack resources for their next presidential nominee, or candidates in high-profile Senate and House races.

A newly formed political action committee called End Citizens United, named for the Supreme Court decision that opened the door to more corporate money in elections, raised about $25 million, largely from small donors, during the 2016 campaign, and is on pace to exceed that amount this election cycle. Most of its spending supports House and Senate Democratic candidates favoring campaign-finance reform.

In other cases, the loss of union money may be offset by grass-roots activism.

In 2016, the four major public-sector unions gave nearly 15 percent of the $17 million raised by the Democratic Legislative Campaign Committee, which funds state legislative races nationwide. But the group is optimistic that any loss of union revenue can be offset by what it calls the “people power” of union members and other voters.