The industry is running ads and op-eds touting private equity-backed jobs in Iowa and Ohio and pouring tens of thousands of dollars in contributions into the election campaigns of Warren's rivals in the Democratic primary.

The success of the Popeyes chicken sandwich "was all possible because of private equity," Drew Maloney, CEO of the industry trade group the American Investment Council, wrote in a Fox Business op-ed this month praising Popeyes and its lead investor, 3G Capital.

The campaign comes as Warren is raising even deeper suspicions about the industry’s practices of buying, selling or sometimes shuttering companies in so-called leveraged buyouts — where the firms borrow large amounts of money to take over struggling companies, loading the takeover targets up with debt.

The distrust is fueled by recent mass layoffs at private equity-backed retailers like Toys "R" Us plus concerns that other industry-owned companies are abusing consumers across the economy. Private equity is also helping fuel consolidation in the beleaguered newspaper industry, with a major union warning this month that the merger of GateHouse Media and Gannett will sacrifice jobs and journalism "on the altar of private equity."

“Washington is finally starting to wake up to the fact that private equity firms are making piles of money while leaving American workers and consumers holding the bag.”

“Washington is finally starting to wake up to the fact that private equity firms are making piles of money while leaving American workers and consumers holding the bag," Warren, a Massachusetts Democrat and presidential candidate, told POLITICO.

Private equity’s growing economic influence is putting its practices under the microscope for problems ranging from the cost of housing to sky-high medical bills, after the industry emerged relatively unscathed from the regulatory crackdown on Wall Street following the 2008 financial crisis.

In July, Warren introduced sweeping legislation endorsed by fellow presidential candidates that would put private equity firms on the hook for problems at companies they take over, including providing more protection for workers if the companies fail.

Warren is now attracting allies to her cause through a wide-ranging investigation by her office into several industries where private equity controls major players — a probe that's partly intended to help advocates on other issues like health care and consumer protection see the bigger role being played by the leveraged buyout firms.

She plans to publish a report based on her findings, an aide said, and her office has so far found evidence that prices for goods and services sold by private-equity backed firms generally rise after takeovers and quality declines. On Tuesday, the House Financial Services Committee will highlight similar concerns and amplify Warren's themes in a hearing — "America for Sale?" — focused on private equity practices.

But it's not just Warren and her allies that the industry has to fear.

The top Democrat and Republican on the House Energy and Commerce Committee are investigating private equity-linked physician staffing companies at the heart of a growing controversy around unexpected medical billing — a probe that's taking aim at two of the biggest PE firms, Blackstone and KKR.

House E&C Chairman Frank Pallone (D-N.J.) and ranking member Greg Walden (R-Ore.) launched the investigation into the private equity companies after it was revealed that the businesses were behind an ad campaign opposing their legislation to end the shock billing. Walden said some of the firms "seem to be making a lot of money off the backs of American patients who play by the rules."

"Right now, we’re focused on ending these outrageous surprise billing practices that these firms are profiteering off of, but I am certainly open to looking into their practices in other parts of our health care system in the future," Pallone said.

Stoking lawmakers' concerns has been an effort by laid-off workers to demand action from Congress and the private equity firms themselves. Hill aides give credit to the labor advocacy group United for Respect that has organized former employees of Toys "R" Us and other struggling retailers controlled by private equity firms. Americans for Financial Reform, a coalition that includes the AFL-CIO, has made private equity legislation a top priority.

Among the lawmakers moved by the stories of laid-off retail workers is Sen. Tammy Baldwin (D-Wis.). She signed on to Warren's plan to crack down on the industry after Shopko, a Wisconsin-based retailer, filed for bankruptcy in January following a private equity takeover in 2005.

Baldwin said private equity firms have made sustainable, job-creating investments in companies in Wisconsin but she called the Shopko buyout and ensuing layoffs "predatory."

"We need to rip up the predatory playbook that private equity firms are using to leave workers with nothing but pink slips," she said.

Such criticism incenses the industry.

"Our private equity portfolio companies added over 100,000 net jobs during our ownership in the past 15 years, underlying their good performance," said Stephen Schwarzman, CEO and co-founder of Blackstone, in October.

The American Investment Council said it’s “important to consider the industry’s full record instead of focusing on single anecdotes."

“You’re cherry picking over a 30-year period a handful of things they say they don’t like,” added one senior official at a major private equity firm who declined to be identified. “You need to step back and think about the airlines dumping their pensions plans in the 2000s. The auto bailouts. The S&L crisis. Private equity has not cost the taxpayers a dime.”

In addition to ads and op-eds, the American Investment Council released research that it produced with Ernst & Young touting private equity’s economic benefits. And the U.S. Chamber of Commerce is trying to bolster the case with its own study that warns of grave consequences from Warren’s legislation.

“Millions of jobs would be lost,” Chamber Executive Vice President Tom Quaadman told reporters on a call with the professor who drafted the business group's report.

A recent academic study by researchers from the University of Chicago, Harvard Business School and other institutions found mixed results of private equity takeovers — significant job losses following buyouts of companies traded on public stock markets but an uptick in employment after buyouts of privately held companies.

"This conclusion cast doubts on the efficacy of 'one-size-fits-all' policy prescriptions for private equity," they said.

Warren’s bill is such an affront to the private equity industry because it would fundamentally change how it does business. When the bill was being developed, Warren’s staff at her insistence spent last fall and winter trying to peel back the layers of the industry, an aide said. The focus of the legislation became the idea that PE firms have authority over their target companies but lack responsibility for the effects of their actions, creating the potential for moral hazard and abuses.

Under Warren’s bill, private equity firms would be liable for debt that’s imposed on companies they control, the pension obligations of the companies as well as government litigation the companies face. The legislation is unlikely to become law but the fact that it's gaining steam at all is rattling private equity.

“The elimination of the concept of limited liability for one particular method of investing and shredding the corporate veil is wildly dangerous,” said the senior private equity representative. “The idea you would familiarize that and desensitize it is a frightening thing.”

It has also caused concern among major investors that have benefited from returns provided by private equity funds.

The Institutional Limited Partners Association, which represents pension funds, endowments and insurance companies, is pushing for greater investor protections in the private equity industry. But the group is concerned that Warren's proposal would deter the firms from continuing to take the risk of running their funds or would cause them to hike fees to pay for insurance policies to protect themselves from greater liability.

"The things we're concerned about are things that would make it potentially difficult for a private equity firm to operate," said Christopher Hayes, the group's senior policy counsel.

It's a worry that's already being heard by lawmakers who are otherwise supportive of Warren's plan.

Sen. Sherrod Brown (D-Ohio), who is co-sponsoring Warren’s bill and wants to probe private equity's role in the housing industry, said in an interview that he was “not sure” about some of the liability issues raised by her legislation and that he wanted to think more about it.

If Democrats win back the Senate, Brown would be in line to chair the Banking Committee, which oversees financial markets as well as the regulators tasked with policing private equity.

“People see abuses, they see the pluses of it,” he said of private equity. “And I’ve talked to people who can give you chapter and verse of how it’s benefited companies and communities and made them more efficient and helped them grow when they couldn't find traditional financing. But you also hear when they come in and they put people out of work in the name of efficiency, but not really for that reason. I just want to know more."