CNN’s Lou Dobbs has been drawing a lot of attention lately for his passionate arguments in favor of stronger immigration enforcement. In a New York Times profile on Wednesday, Dobbs told the paper, “I know whereof I speak on the political economy. I don’t come to a conclusion out of thin air because of some partisan or ideological viewpoint, but rather with an analysis of the facts.”

But on the special Wednesday edition of Lou Dobbs Tonight originating from Cancun, Mexico — trumpeted in advance by CNN as “Three heads of state and one tough reporter who says it’s time for answers [on immigration]. Lou Dobbs is crossing the border in hot pursuit of those answers” — the anchor got careless with one of the central facts of the debate: Exactly how many immigrants are in the U.S. illegally.

Dobbs first referred to “the 11 to 20 million illegal aliens in the United States,” and then, squaring off against Hector Flores, president of the League of United Latin American Citizens, he said, “Let’s talk about the reality of it. … The number, 11 million, 20 million, by one study. Should there be amnesty for all of them?” Next he asked viewers to vote in an online poll: “Do you believe that the illegal employers of the 11 to 20 million illegal aliens in the United States should be granted amnesty?”

After reiterating his view that “there are as many as 20 million illegal aliens living in the United States,” mostly immigrants from Mexico, Dobbs introduced his final guest, John Trasvina of the Mexican-American Legal Defense and Education Fund, who, as Dobbs aptly put it, “says no one responsible estimates the number of illegal aliens that high”:

Dobbs: John, good to have you with us.

Trasvina: Hello, Lou. Being in Cancun has mellowed you some. It’s a good thing.

Dobbs: It is a very good thing. It hasn’t, however, changed the facts or the issues or the critical and urgent nature of them. John, how many illegal aliens do you estimate, since you reject the Bear Stearns study that extrapolated that as many as 20 million illegal aliens last year were living in the United States?

Trasvina: Well, the most authoritative study says 11 million or 12 million. … So 20 million is very exaggerated.

As the sun set in Cancun, Dobbs responded with an even more blasé treatment of the numbers.

“And the scale … 12 million, 20 million, 15 million,” he said, throwing up his hands both literally and figuratively. “The fact is that is a huge number of people to be in our country without the permission of this government representing American citizens.”

Dobbs gets half a point for at least stating that there is a range of estimates. But which one is the best?

Estimating the number of immigrants in the U.S. illegally is an inexact science, to be sure. But Dobbs seems quite alone in his belief that “there are as many as 20 million illegal aliens living in the United States.” Every major newspaper in the country that has reported a number over the past several days — from the Wall Street Journal (subscription required) to the Los Angeles Times to the Boston Herald to the St. Louis Post-Dispatch — has given an estimate of about 11 million or 12 million. Not one has produced a higher number supporting Dobbs.

Most papers reporting the story haven’t given a source for the estimate; however, it does line up with the population statistics of the government and with a study by the nonpartisan Pew Hispanic Center. But Dobbs’ uncritical embrace of Bear Stearns’ 20 million figure is more confounding.

In a January 2005 paper (200K PDF), Bear Stearns dismissed the government’s estimate as “incomplete,” looking instead at “several micro trends at the community level.” Based on a high number of border crossings and on “very dramatic increases” in the usage of social services such as school enrollments, foreign remittances, and housing permits in a small sample of immigrant gateway communities, Bear Stearns, applying the high rate of growth it saw in those areas to the rest of the country, argued that the number of illegal immigrants “may be as high as 20 million people.”

The study was heavy on extrapolation and light on the large-scale demographic data needed to prove such a contrarian assertion. Common sense suggests that the number of illegal immigrants in the country would tend to be underestimated, but Bear Stearns’ analysis is far from convincing, or conclusive.

We are inclined to believe the much more comprehensive analysis of the Pew Hispanic Center, which reported there were 11.1 million unauthorized migrants in the U.S. in March 2005 and (in perhaps the authoritative study Trasvina referred to) gave “an estimate of 11.5 to 12 million for the unauthorized population as of March 2006.”

Dobbs’ suggestion that somehow Bear Stearns’ estimate is just as valid as the better-grounded lower estimates (“11 to 20 million”) just doesn’t stand up to close scrutiny.