INDIANAPOLIS | U.S. Sen. Joe Donnelly, D-Ind., is the fourth-most bipartisan senator to serve over the past two decades and the Democrat most willing to work across party lines, according to an analysis from a think tank led by former U.S. Sen. Dick Lugar.

The Lugar Center recently expanded its Bipartisan Index to rate U.S. senators based on how often legislation they sponsored from 1993-2014 attracted co-sponsors from the other political party, and whether a senator co-sponsored measures proposed by someone not of his or her party.

Former U.S. Sen. Lincoln Chafee, R-R.I., who briefly ran for the Democratic presidential nomination, topped the index as the most bipartisan senator. He was followed by U.S. Sens. Susan Collins and Olympia Snowe, both of Maine.

Donnelly, who is in the fourth year of his first term, came in fourth out of the 227 senators who have served in the 100-member chamber since 1993.

"My responsibility is to do what is right for Hoosiers no matter which party an idea may come from," Donnelly said.

"I believe we are stronger and more effective when we work together, and when we prioritize bipartisan efforts we can get things done that will help Indiana and our country."

The namesake of the Lugar Center placed 24th for bipartisanship, just ahead of former Connecticut U.S. Sen. Joe Lieberman, the 2000 Democratic vice presidential nominee.

Lugar represented Indiana in the Senate from 1977 to 2013. He said he believes the nonpartisan analysis "illustrates the changing nature of the Congress" and explains in part "why it has become so dysfunctional in recent years.

"But it also shows that some lawmakers with strong ideological views can nonetheless find common ground with members of the other party," Lugar said.

Of the two senators who also represented Indiana since 1993: Democrat Evan Bayh rated 61st on the Bipartisan Index; Republican Dan Coats placed 214th for his work during two nonconsecutive six-year terms.

Two Republicans from Illinois were among the most willing to work with Senate Democrats. Incumbent U.S. Sen. Mark Kirk and former U.S. Sen. Peter Fitzgerald (1999-2005) were rated 18th and 22nd, respectively.

Former U.S. Sen. Carol Moseley Braun, D-Ill., (1993-1999) was the highest-rated Illinois Democratic senator at 93.

She was followed by Paul Simon (1985-97) at 120; Dick Durbin (1997-present) at 126; Barack Obama (2005-08) at 165; and Roland Burris (2009-10) at 221.

The Lugar Center determined the least bipartisan senator over the past two decades was former U.S. Sen. Jim DeMint, R-S.C.

DeMint quit Congress in 2013 to become president of the Heritage Foundation, a leading conservative think tank.

His replacement, U.S. Sen. Tim Scott, R-S.C., was the second-least bipartisan senator, followed by U.S. Sen. Mike Lee, R-Utah, and U.S. Sen. Ted Cruz, R-Texas.

Former U.S. Sen. Hillary Clinton, D-N.Y., scored highest (156) for bipartisanship among current and former Democratic senators still running for president this year. At 217, Vermont Sen. Bernie Sanders was the third-least likely Democrat to work with Republicans.

The Republican presidential candidate most willing to cross party lines was U.S. Sen. Lindsey Graham, R-S.C., in 122nd place.

He was followed by former U.S. Sen. Rick Santorum, R-Pa., at 145; U.S. Sen. Marco Rubio, R-Fla., at 170; U.S. Sen. Rand Paul, R-Ky., at 222; and Cruz at 224.

Edward Montgomery, dean of the McCourt School of Public Policy at Georgetown University, which co-sponsors the Bipartisan Index, said he hopes it reminds lawmakers that conservative or progressive ideologies shouldn't be a deterrent to political cooperation.

"Our society faces significant challenges both domestically and abroad," Montgomery said. "Now, more than ever, we need our lawmakers to work together to get things done."

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