Each year, the National Journal comes out with a list of the most liberal and conservative members on Capitol Hill, based on their voting records in the past year.

This year, Sen. Tom Udall (D-New Mexico) came in a tie for fifth among the 100 members of the US Senate for most liberal while the only Republican in New Mexico’s delegation — Rep. Steve Pearce – came in as the 36th most conservative of the 435 members of the House of Representatives.

Sen. Tom Udall (D-New Mexico) at news conference 11/1/11

In what might be a bit of surprise, Rep. Ben Ray Luján received a higher liberal ranking (108) than Rep. Martin Heinrich (146).

The fifth member of the New Mexico delegation — retiring Sen. Jeff Bingaman – came in as the 25th most liberal.

Since 1981, The National Journal comes up with two separate indices each year — a liberal rating and conservative rating — based on selected roll-call votes:

The liberal percentile score means that the member voted more liberal than that percentage of his or her colleagues in that issue area in 2011. The conservative figure means that the member voted more conservative than that percentage of his or her colleagues.

For example, a House member in the 30th percentile of liberals and the 60th percentile of conservatives on economic issues voted more liberal than 30 percent of the House and more conservative than 60 percent of the House on those issues, and was tied with the remaining 10 percent. The scores do not mean that the member voted liberal 30 percent of the time and voted conservative 60 percent of the time.

… Critics have sometimes accused National Journal of rigging the vote ratings so that certain members of Congress are ranked as the most liberal or most conservative. The criticism is unfounded. When we select the votes, we don’t have any idea how an individual member of Congress will be ranked.

No single measure of voting behavior is likely to be perfect. For instance, some House Republicans occasionally voted against budget-cutting measures last year because they didn’t think the bills reduced spending enough. In so doing, they voted against the overwhelming majority of House conservatives and with the overwhelming majority of House liberals (who opposed the measures because they thought the bills cut spending too much). In such cases, their votes were counted as liberal because they voted with liberals. It’s beyond the capacity of a vote ratings system to determine why a member voted the way he or she did on any particular piece of legislation.

With all those caveats, here’s raw data from the New Mexico delegation: (Click on the chart to view it unobstructed.)

In regards to Udall, he finished in a 5-way tie for fifth place on the most liberal list, joining Barbara Boxer of California, Sherrod Brown of Ohio, Tom Harkin of Iowa and Barbara Mikulski of Maryland.

The Journal wrote of Udall: “He belongs to a political clan well known in the West and nationally that is sometimes called the ‘Kennedys of the West.’ Udall added the Foreign Relations Committee to his workload in 2011, expressing a desire to help ratify the Law of the Sea Treaty defining the international usage of oceans. The treaty has drawn fierce opposition from conservatives concerned about its infringement on U.S. sovereignty.”

In the past year, Sen. Udall has also garnered attention for calling for a change in the filibuster rules and seeking a Constituional amendment to overturn the Citizens United Supreme Court ruling.

Rep. Steve Pearce (R-New Mexico)

On the conservative side, Rep. Pearce has taken a high-profile stance opposing placing the dunes sagebrush lizard on the endangered species list, claiming that doing so would harm oil and gas interests in southeastern New Mexico.

Last year, Udall was ranked as the 15th most liberal senator; Bingaman was 25th. In the House of Representatives, Luján was rated the 106th and Heinrich 151st on the liberal scale.

Overall, the National Journal rankings this year confirm the charges that Capitol Hill has become more polarized in recent years. As one of its staff writers pointed out:

For the second year in a row but only the third time in the 30 years that National Journal has published these ratings, no Senate Democrat compiled a voting record to the right of any Senate Republican, and no Republican came down on the left of any Senate Democrat.

… Believe it or not, it wasn’t always so. In 1982, when National Journal published its first set of voting ratings, 58 senators—a majority of the 100-member chamber—compiled records that fell between the most conservative Democrat (Edward Zorinsky of Nebraska) and the most liberal Republican (Lowell Weicker of Connecticut). Now it’s zero, zip, nada.

You can read the National Journal article by clicking here. The magazine’s website has plenty of links to the data as well as charts of the ratings nationally.