NEW YORK — Once a key Republican constituency, doctors have shifted their campaign contributions to Democrats as more women enter the field and more doctors move from being small business owners to hospital employees.

The percentage of physicians donating to Republicans has fallen from almost 75 percent in 1996 to less than 50 percent in 2012, according to an analysis of campaign contributions published Monday in JAMA Internal Medicine. Total contributions by doctors have increased about sevenfold over that time.

The support of more doctors may help Democrats win over voters and fend of criticism from Republicans that the Affordable Care Act is bad for patients and the practice of medicine, said Robert Blendon, a professor of health policy at the Harvard School of Public Health. The backing of women doctors could be particularly helpful in seeking key female swing voters.

“You are going to have physician voices on TV and radio supporting Democratic presidential candidates who want to expand the role of government,” Blendon said in a telephone interview. “Republicans won’t have a monopoly on having physician leaders always on their side. Having more women leaders in medicine speaking on behalf of Democrats is going to give them more credibility.”

Campaign contributions by physicians increased to $189 million in 2012 from $20 million in 1992. About 9.4 percent of doctors made a donation greater than $200 in 2012 compared with 2.6 percent in 1992, the study found.

For much of the 1990s, total donations to both parties combined were about $20 million to $30 million per election cycle before jumping in 2004 during President George W. Bush’s re-election to $87 million and to $117 million in 2008 when President Barack Obama was elected.

The data include federally reported campaign contributions to presidential and congressional candidates and partisan organizations such as party committees and super political action committees from 1991 to 2012.

While campaign contributions in the general population have also been moving more toward Democrats, the trend has been greater among physicians during the last three election cycles, said David Rothman, a study author and director of the Center for the Study of Medicine as a Profession at Columbia University in New York.

A key factor in the rise in Democratic support is the increase in women physicians, Rothman said.

Almost half of medical school graduates in 2012 were women compared with about a third in 1982, according to the Association of American Medical Colleges.