Beginning Wednesday, the Service Employees International Union, one of the country’s biggest unions, will call upon people to attend protests on July 17 in 100 cities in 25 countries. The rallying cry will be: Take back the economy from buyout firms that the union says have exploited tax loopholes to amass great wealth at others’ expense.

“We think the buyout industry and the way it operates are systematic of what’s wrong in this economy,” said Stephen Lerner, director of the union’s private equity project. “We want to make them responsible corporate citizens.”

The S.E.I.U.’s latest effort is an escalation of a fight that began last April, when it began a broad campaign against private equity firms with a study questioning the value that the leveraged buyout industry adds to the national economy.

S.E.I.U. officials acknowledge, however, that changing the tax code could upend the modern corporate regime and say they have not endorsed any specific proposals.

The union also argues that the attention on private equity firms has been justified by the huge role they now play in the economy. Companies owned at least in part by Kohlberg Kravis employ more than 816,000 people, according to the firm’s Web site — more than the population of San Francisco.

The fight has been contentious at times, notably during an altercation between an S.E.I.U. member and Mr. Rubenstein at an industry conference in Philadelphia. And the bulk of the S.E.I.U.’s campaign has taken place amid the credit squeeze, which has all but squelched private equity’s lifeblood of striking deals.

To the union, however, that same economic malaise has hurt ordinary workers more.

And in tune with the times, the union plans to introduce a political angle in its attack against private equity: The S.E.I.U. will highlight the fund-raising that Mr. Kravis, a longtime Republican donor, has done for Senator John McCain, that party’s likely presidential nominee. The goal, S.E.I.U. officials say, is to highlight the unwillingness of Mr. McCain to consider revamping the tax code and make a partisan appeal to a possible Democratic administration and more Democratic Congress.