The first rigorous look at donor doctors also finds they've become increasingly generous, with political contributions surging to almost $200 million in recent years.
An increase in female doctors who more often than men donated to Democrats and a decline in physicians working on their own or in small practices occurred during study years. Those changes likely contributed but reasons for the political shift are unclear, said study co-author David Rothman, a social medicine professor at Columbia University's medical school.
"We've got to stop thinking of physicians as a group as 'solidly Republican.' They are polarized, almost equally divided between Republicans and Democrats," Rothman said.
The study focused on donations of $200 or more to presidential and congressional candidates or political organizations from 1991 through 2012. At the beginning, almost 3 percent of U.S. doctors made contributions, rising to almost 10 percent by the end of the study.
Doctor donations to Republican candidates peaked in the mid-1990s, when almost 75 percent of all MD contributions went to the GOP. Those donations mostly declined after that, to about 50 percent in 2011-12. The exception was in 2009-10 during emergence of the Affordable Care Act, when Republican donations briefly increased.
Donations from non-physicians also increased and tilted more Democratic during the study, but the authors say the MD findings are remarkable for two reasons: they defy the historical image of doctors as a conservative, right-leaning bunch, and political contributions from doctors increased at a greater rate than among the general public.
Political alliances also differed by medical specialty: Surgeons, dominated by men, were the strongest GOP supporters while pediatricians more than half of whom are women were most likely to contribute to Democrats.
By the end of the study, 24 percent of women who donated gave to GOP candidates versus 52 percent of the men.
Women comprise almost one-third of the nation's 1 million physicians and almost half of medical school graduates, according to 2012-13 data from the Association of American Medical Colleges.
The study was published Monday in the medical journal JAMA Internal Medicine. The researchers analyzed data from the Federal Election Commission; a political contributions database created by Stanford political scientist Adam Bonica, the lead author; and health-care provider directories.
Doctors' right-leaning reputation dates back at least as far as 1965, when the American Medical Association strongly opposed the passage of Medicare, the study authors say.
The AMA has since lost sway less than one-third of U.S. physicians are members but it remains the nation's largest doctor group and a powerful lobbying presence in Washington. It also has moved more to the center, including voicing support for the Affordable Care Act.
For the 2012 elections, the AMA says reports to the FEC show it contributed $2 million to campaign committees, with recipients including the Democratic Congressional Campaign Committee and the Republican National Committee.
A journal editorial says the study provides an unprecedented, though largely predictable description of doctors' campaign contributions. The author, Dr. Arnold Relman, a professor emeritus at Harvard's Brigham and Women's Hospital, also questioned whether the study represents all physicians since contributions totaling less than $200 weren't included.
Rothman said it's unlikely the smaller contributions would have changed the results.