The most bipartisan lawmakers in the Senate last year were mostly Republicans, while Democrats were ranked least likely to see eye-to-eye with those across the aisle, according to a study released Monday.

Sen. Susan Collins of Maine was ranked the most bipartisan senator, and she was followed by two other Republicans, Rob Portman of Ohio and Shelley Moore Capito of West Virginia, according to the Lugar Center and Georgetown University's McCourt School of Public Policy biannual study.

Sens. Joe Donnelly of Indiana was the only Democrat among the top 10 members.

The other Republicans rounding out the top 10 were Sens. Dean Heller of Nevada, Lindsey Graham of South Carolina, Chuck Grassley of Iowa, Cory Gardner of Colorado, Todd Young of Indiana, and Marco Rubio of Florida.

At the bottom of the list, eight of the 10 least bipartisan legislators were Democrats. Vermont Sen. Bernie Sanders, an independent who caucuses with the Democratic Party, was deemed the least bipartisan person in the upper chamber.

Sanders was followed by Sens. Chris Murphy, D-Conn.; Richard Shelby, R-Ala.; Patty Murray, D-Wash.; Luther Strange, R-Ala.; Al Franken, D-Minn.; Ed Markey, D-Mass.; Dick Durbin, D-Ill.; Chris Van Hollen, D-Md.; and Jeff Merkley, D-Ore.