WASHINGTON — It's a line you'll hear many times from Sen. John McCain over the next few months: "In just a few years in office, Sen. Obama has accumulated the most liberal voting record in the Senate."
But McCain's campaign also pointed out recently that Barack Obama votes with President Bush about half the time.
So the biggest liberal in the Senate is a solid supporter of President Bush? What gives?
Actually, there's some truth to both claims. The political magazine National Journal rated Obama the most liberal senator for 2007, while Congressional Quarterly calculated that Obama voted with Bush 40 to 50 percent of the time the past two years.
McCain's seemingly contradictory claims illustrate the limitations of congressional ratings.
"They can give a good inkling of whether someone is liberal or conservative, but they're not good enough for a precise determination of what someone's ideology is," said Josh Clinton, a political science professor at Princeton University.
'Obama the liberal'
It's a standard technique from the Republican playbook: brand Democratic opponents as "tax-and-spend liberals."
When National Journal released its vote ratings in January, it gave Republicans fresh ammunition to use the term against Obama. The nonpartisan magazine, which in 2004 had determined that the Senate's No. 1 liberal was also a presidential candidate — John Kerry — published a story headlined "Obama: Most Liberal Senator in 2007."
Republicans have been reciting the line ever since. Yet other ratings paint a different picture.
Voteview.com, a site created by political scientists that plots lawmakers on a liberal-conservative scale based on their voting patterns, calculated that nine senators were more liberal than Obama in the current Congress.
"Obama is a liberal, but he's not the most liberal," said Keith Poole, a University of California at San Diego professor who runs the site. By comparison, McCain is the eighth most conservative.
Ratings from Congressional Quarterly, an affiliate of the St. Petersburg Times, also provide a mixed picture.
In CQ's calculation of party unity, which measures how often members vote with their party on bills where the parties split, Obama got a 97 percent rating last year. Ten Democrats had higher scores.
On votes where Bush indicated his position, CQ found Obama supported the Republican president 40 percent of the time in 2007. That 40 percent rating put Obama in the middle of the pack for Democrats. In 2006, Obama voted with Bush 49 percent of the time.
McCain had the Senate's highest presidential support score last year, 95 percent, but he missed more than half of the votes because he was campaigning. And McCain hasn't always been such a strong backer of Bush. He supported Bush 77 percent in 2005 and has averaged 89 percent since 2001.
Some math, judgment
National Journal, Voteview and CQ use very different approaches.
National Journal relies largely on the judgment of its editors and reporters. They choose votes that they believe are important or show ideological distinctions (they included 99 of the 442 Senate votes last year), and they decide which side in the vote is liberal and which is conservative.
"We're trying to pick votes where some ideological differences are displayed and show how members of Congress line up relative to one another," said Charles Green, editor of the magazine.
CQ takes a more empirical approach and calculates how often members vote with their party or the president.
"We don't try to establish a litmus test or ideological label," said John Cranford, CQ's national editor.
Voteview uses a complicated calculation based on patterns of how often each member of Congress votes with other members and plots each member on a liberal-conservative spectrum.
Labor unions and advocacy groups do their own congressional ratings, usually based on a small number of votes on their key issues. Typically, the groups alert lawmakers when a vote will be included in their ratings, which, depending on the heft of the group, can influence the way some lawmakers vote.
"They are not picking votes in an objective way to assess where a member is on an issue," said Clinton, the Princeton professor. "They want to exercise leverage."
Political scientists caution that all ratings have limitations — the biggest being that they are based on the congressional agenda, which is set by the party in control. That can skew the votes so they don't fairly represent a member's ideology.
For example, a senator who is liberal on social issues but conservative on economic topics can have his or her overall "score" distorted if there are many votes on social-issue bills but few on economic ones.
Likewise, missed votes can weaken the value of the results. Because Obama was campaigning, he missed 33 of 99 votes that National Journal rated. McCain missed so many the magazine didn't give him an overall rating.
Obama complained about the way National Journal categorizes votes as liberal or conservative. He said he disagreed, for example, with the judgment of the magazine that his vote to establish a Senate office of public integrity should be considered a liberal vote.
The Voteview approach is widely praised by political scientists because it has been accurate at predicting how members vote. But Poole acknowledges that his liberal-conservative scale doesn't capture the complexity of lawmakers' viewpoints.
"American politics is a philosophical mess in that there is no philosophical coherence to political parties. The parties are just groups of issue positions," he said.
Green, the National Journal editor, says voters shouldn't rely on a single rating to determine a candidate's ideology. "There's pluses and minuses to each rating system. If you look at a number of them, I think you have a pretty good picture."
Thomas Mann, a senior fellow at the Brookings Institution, a political think tank, said they have limited value.
"This is a case," he said, "where statistics can do more harm than good."
Times researcher Angie Holan contributed to this report. Bill Adair can be reached at firstname.lastname@example.org or (202) 463-0575.
"In just a few years in office, Sen. Obama has accumulated the most liberal voting record in the Senate."
John McCain, June 3 in a speech in New Orleans
McCain suggests it is a cumulative rating for all of Obama's time in the Senate. But it's one rating for one year. Measurements for other years and by other groups show Obama is not the No. 1 liberal — in some cases, far from No. 1.
on the Web: For more rulings on the candidates' statements, go to Politifact.com