Sen. Joe Manchin, D-W. Va., left, and Sen. Susan Collins, R-Maine, who serve together on the Senate Appropriations Committee and the Senate Intelligence Committee, confer March 28, 2017, on Capitol Hill after doing television news interviews, in Washington.

In an era of often bitter partisanship, U.S. Sen. Susan Collins stands out as the premier example of a senator willing to reach across party lines time and again.

The Lugar Center and Georgetown University’s McCourt School of Public Policy picked the Maine Republican as the most bipartisan member of the Senate for the fourth straight year.

“Senator Collins continues to set the gold standard for bipartisan productivity in the U.S. Congress,” former Indiana Sen. Richard Lugar said in a statement released Wednesday.

“Her leadership of the Bipartisan Index shows a deep devotion to working with both parties for the interests of Maine and our country,” he said.

Collins said that “given how divided our country has become, I feel a special obligation to try to find a path forward on the many contentious issues that we are facing.”

A couple of months ago, speaking to a No Labels conference, Collins bemoaned the way the ground under middle-of-the-road politicians is “melting like late-winter snow in Maine.”

“In today’s highly charged partisan atmosphere, it is more difficult than ever to find common ground,” Collins said in a press release Wednesday.

Maine is one of only a few states whose senators have always had a positive bipartisan ranking from the Lugar Center. Collins and former U.S. Sen. Olympia Snowe are among the three least partisan senators since the rankings began a quarter century ago.

U.S. Sen. Angus King, a first-term Maine independent, ranked 25th in the Senate for his bipartisanship.

The most partisan member of the Senate in the 2015-16 session was Vermont independent Bernie Sanders. He easily beat out Texas Republican Ted Cruz for the last place showing.

The only senator who came anywhere near Collins in terms of bipartisanship was Joe Donnelly, an Indiana Democrat.

The Bipartisanship Index, issued annually, bills itself as “a nonpartisan ranking of how often each member of Congress works across party lines based on sponsorships and co-sponsorships of legislation.”

It looks at the bills that each member sponsors or co-sponsors to see how often senators sign on to measures with those from another party, an indication of their readiness to go outside their own party lines to pursue long-term priorities.