The Most and Least Bipartisan U.S. Senators, Measured Two Ways

Two rulers, one conclusion at the top

Bipartisanship gets asserted in speeches. It is also countable. The Lugar Center and Georgetown's McCourt School count it one way, scoring bill sponsorship and cosponsorship against two decades of benchmarks; GovTrack counts it another, as the plain percentage of a member's cosponsorships that go to the other party's bills. Both rulers put the same three names on top: Susan Collins, Gary Peters, and Maggie Hassan (Lugar Center-McCourt School, 2024; GovTrack, 2024 Report Cards).

Collins's numbers are extreme by any standard. Two thirds of her cosponsorships in the 118th Congress, 66.7 percent, went to Democratic bills, and she missed zero votes across the entire Congress (GovTrack, 2024 Report Cards). The Lugar Center has now ranked her the most bipartisan senator nine times in eleven years (Collins Senate office, citing Lugar Center-McCourt School, 2024).

What the Bipartisan Index actually counts

The Lugar Center built its instrument to be hard to game. It scores only two behaviors, both recorded permanently in the Congressional Record: whether the bills a member introduces attract cosponsors from the other party, and whether the member puts their own name on bills introduced across the aisle. Press releases, floor speeches, and votes do not count, because the Center's founders judged sponsorship behavior the cleanest paper trail of actual cross-party work. Each member's score is benchmarked against twenty years of historical congressional data, so the Index measures a member against the institution's own norms rather than against a shifting news cycle (The Lugar Center, Bipartisan Index methodology).

The edition cited throughout this report is the single-year 2023 edition covering the first session of the 118th Congress, released jointly with Georgetown's McCourt School in May 2024; it is the most recent edition available as of July 2026, and this page names it precisely because a ranking that cites an index without citing its edition cannot be checked (Lugar Center-McCourt School, May 2024). The GovTrack cosponsorship figure that orders the table below it is a different, simpler instrument: the raw percentage of each senator's cosponsorships that went to other-party bills across the full two-year Congress, computed as of February 13, 2025 (GovTrack, 2024 Report Cards). Where a precise Lugar rank is documented for a senator, the blurb reports it. Where it is not, this page does not invent one.

The Texas split

The same state can send both ends of the spectrum to Washington. The Lugar Center flagged Texas as the largest same-state split in the Senate: John Cornyn ranked 5th on the 2023 Bipartisan Index while Ted Cruz ranked 89th (Lugar Center-McCourt School, 2024). GovTrack's full-Congress data shows the same shape: 39.1 percent of Cornyn's cosponsorships crossed the aisle against 18.3 percent for Cruz (GovTrack, 2024 Report Cards). Party does not explain the difference. The two men share one.

The pattern repeats across the table. The top fifteen contains eight Republicans and seven Democrats. The bottom ten contains eight Democrats and two Republicans, but the 2023 Lugar edition's very lowest scorers included Republicans Katie Britt and Eric Schmitt. Neither party owns cross-party work, and neither party owns the refusal of it.

Bipartisanship is not the same as effectiveness

Jeff Merkley demonstrates that the two virtues measured on this site are different. He sits 72nd of 80 in cross-party cosponsorship at 16.2 percent, yet the Center for Effective Lawmaking ranks him the fifth most effective Senate Democrat with five laws enacted (GovTrack, 2024 Report Cards; Center for Effective Lawmaking, 2025). The reverse case exists too: Thom Tillis cosponsored across the aisle 41.7 percent of the time and got zero sponsored bills enacted. Working with the other party correlates with lawmaking success in CEL's long-run research, but it does not guarantee it in any single Congress.

Floor leaders deserve one caveat, stated plainly: by custom, party leaders sponsor and cosponsor few bills, so Chuck Schumer's 74th-place figure measures a job description as much as a disposition. The number is real; its interpretation has limits.

The geography of crossing the aisle

The top of the table has a shape, and it is geographic as much as personal. Collins of Maine, Murkowski of Alaska, Hassan of New Hampshire, Peters of Michigan, Warnock of Georgia, Kelly of Arizona, and Rosen of Nevada all represent states their party cannot take for granted, and every one of them cosponsors across the aisle at rates far above the chamber median of 26.4 percent (GovTrack, 2024 Report Cards). The bottom of the table is dominated by senators from states that have not been seriously contested in decades: Massachusetts, Oregon, Hawaii, Washington, New York, and Utah. The data cannot prove which way the causation runs, and this page does not claim it can. It records the correlation and lets the reader weigh it.

The correlation is not a law. Grassley's Iowa and Boozman's Arkansas are safe seats, and both men sit in the top twelve anyway. Warnock won Georgia by two points and could have chosen a fortress strategy; 53.1 percent of his cosponsorships went to Republican bills instead, the second-highest Democratic figure in the chamber (GovTrack, 2024 Report Cards). Electoral pressure explains the pattern; it does not excuse anyone from it.

What the freshman data warned about

The Lugar Center noted that the eight first-year senators of 2023 dragged down the chamber's overall score, with Katie Britt of Alabama scoring lowest in the Senate and every freshman landing 72nd or worse (Lugar Center-McCourt School, 2024). The 14 senators who arrived in 2025 and 2026 are not yet scored, so this report does not rank them. Whether the newest class repeats that pattern is an empirical question the next Index edition will answer, and this page will be updated when it does.