American doctors have shifted their political support away from Republicans toward Democrats, according to a study released on Monday.

The percentage of contributions that doctors have given to Republicans has mostly declined since 1996. It dipped below 50 percent in 2008 before rebounding in 2010. It then dipped below 50 percent again in the 2012 election. As doctors’ political giving has increased, so has their partisanship. Very few doctors split their contributions between the two major parties: just 3 percent did in the 2012 cycle, down from 5.9 percent in 1992.

The study attributed the partisan shift to an increase in female physicians and the shrinking number of doctors running their own solo and small practices. The study, which examined federal campaign giving by doctors from 1991 through 2012, was conducted by Adam Bonica of Stanford, Howard Rosenthal of New York University and David J. Rothman of Columbia.

The authors found increases in both the number of physicians donating to federal campaigns and in the amounts given. Nearly one in ten American physicians made a federal campaign contribution during the 2012 election, with a total of $143 million, a record for the profession.