Diane Fisher, a registered nurse at an oncology practice in South Jersey, has seen firsthand the impact of expanded health insurance under the Affordable Care Act.

Before the ACA, uninsured patients would show up with late-stage cancer, and "we would have to send them to the hospital for chemo, so they could get charity care," said Fisher, estimating the practice would routinely have 15 to 20 such patients. Now it has just two.

"It's been an obvious change to us," said Fisher, who came to Philadelphia on Saturday to participate in a rally of health-care professionals opposed to the dismantling of the ACA. With a group of 75, she marched from Republican Sen. Pat Toomey's office at 1628 JFK Blvd. to the rally, at Thomas Paine Plaza across from City Hall. The protest, organized by Women's March Pennsylvania,  drew hundreds,  many from the suburbs.

The high-energy gathering came just before Congress heads back into session Monday in Washington, after two weeks of news on how dramatically Republicans would alter the health-care framework established by the ACA.

Republican lawmakers would expand health savings accounts, introduce tax credits based on age rather than income, and cap federal Medicaid payments to states, imperiling expansion of the program that serves the poor.

About 480,000 people in the Philadelphia region have insurance through the federal health-insurance exchange or Medicaid expansion.

"I think we all know people who will suffer if they lose access to coverage," said Anna Olivier, a nurse practitioner at Penn Family Medicine and Community Health in University City and coordinator of the demonstration at Toomey's office.

Similar rallies of health-care professionals, spearheaded by a New York doctor who graduated from Abington High School, were scheduled in 17 other cities nationwide, according to a Facebook organizing page.

Speaking at the rally in Center City, Sen. Bob Casey (D., Pa.) got big cheers by declaring that Republicans do not have a plan but rather a scheme to take health care away from millions.

"We've got a plan. Let's fight them," he urged the crowd.

Before the speeches by activists and politicians started, U.S. Rep. Dwight Evans (D., Pa.) walked through the crowd, encouraged by the persistence of protesters. "I believe this resistance is registering," he said.

Many of Saturday's protesters made the point that they were not paid to be there, as has been suggested of protesters at town-hall meetings held during the  congressional recess.

Chris Hindman, who said she worked for 40-some years as a nurse, and her husband, Paul, wore notes pinned to their jackets that read: "Not paid to be here -- Keep the ACA."

They have a 46-year-old daughter who was diagnosed with cancer in 2009 and was refused insurance until the ACA took effect.

"It's real. It does affect people," said Paul Hindman. "We're concerned about that, and I'm a Republican."

Health-care professionals, out in force Saturday, recounted the benefits of the ACA and said they were worried about losing them.

Alison Purcell, a nurse practitioner at a Fort Washington practice, fears that Republican proposals, such as issuing tax credits to offset the cost of health insurance by age rather than by income, will hurt patients who have gained coverage under the ACA.

"I see a lot of those people losing coverage if there are drastic changes," she said.

Health savings accounts, like Individual Retirement Accounts, help people who have money to put in them, Purcell said: "It's not going to help people who are scraping by."

A Swarthmore psychiatrist, Andrea Belasco, who has a private practice and works in the Widener University Counseling Center, said the ACA rule that allowed children to stay on their parents' insurance until they turned 26 has been a huge help.

"Before the ACA, it was hard to arrange for continued care" after students left college, Belasco said.

After the rally, Olivier, the nurse practitioner at Penn,  said she planned to remain active in the bid to preserve expanded access to care.

"This doesn't feel optional to me," she said. "My job is to provide care to the community. This has become part of it."