Senate Democrats on Thursday rejected a GOP attempt to require next year's census forms to include a question about citizenship status -- an effort opponents called anti-immigrant.

The proposal by Republican Sens. David Vitter of Louisiana and Robert Bennett of Utah was aimed at excluding non-citizens from the population totals that are used to figure the number of congressional representatives for each state.

But the Vitter plan fell after a 60-39 procedural vote made it ineligible for inclusion in a bill funding the census.

The "relevancy" test, called "germaneness," is a very strict one, and often can exclude a measure that is seemingly relevant. The spending bill, for instance, funds the Commerce, Justice, and State Departments; the census is part of the Commerce Department.

Critics said Vitter's plan would discourage immigrants from responding to the census and would be hugely expensive. They also said that it's long been settled law that the apportionment of congressional seats is determined by the number of people living in each state, regardless of whether they are citizens. A separate survey already collects citizenship data.

The Commerce Department estimated that making the change on census forms would have cost the federal government $1 billion, but Vitter, after weeks of requesting supporting data, said the explanation fell short.

"After three weeks of asking for the data behind that $1 billion claim, they sent us one piece of paper with 10 bullet points on it, all very general statements and suggestions, and with the final bottom line, a nice, even, round figure of $1 billion. That's very unimpressive, in my opinion, in terms of any precise accounting for $1 billion," Vitter said.

And citing what he called the Census Bureau's "horrendous record" of cost estimation, Vitter noted a nonpartisan General Accounting Office study that noted: "Given the bureau's past difficulties in developing credible and accurate cost estimates, we're concerned about the reliability of the figures that were used to support the 2010 budget."

Census data is also used to distribute billions of dollars in federal aid.

"The current plan is to reapportion House seats using that overall number, citizens and non-citizens," Vitter said. "I think that's wrong. I think that's contrary to the whole intent of the Constitution and the establishment of Congress as a democratic institution to represent citizens."

If Vitter were successful -- and if non-citizens were excluded from the census count for congressional apportionment -- states with fewer immigrants would fare significantly better in the upcoming allocation of House seats.

States such as California and Texas would fare worse than they would under the current way of allocating seats, which under the Constitution is based on the "whole number of persons" residing in a state.

Louisiana stands to lose one of its seven House seats in the upcoming round of reapportionment. Vitter says that if non-citizens were excluded, Louisiana and eight other states would keep or gain congressional seats that would go to California, Texas, Illinois and New York.

In a floor speech before the vote, Vitter pleaded in vain for support from his colleagues.

"This is an important question, and we simply shouldn't forge ahead when Americans have a fundamental problem with not even asking the citizenship question and with, therefore, forging ahead with the plan to reapportion the entire U.S. House of Representatives, putting noncitizens in the mix when the whole notion of a representative democracy and of Congress is to represent citizens of the country," he said. "I urge my colleagues to support that position."

Sen. Barbara Mikulski, D-Md., managing the spending bill on the floor, said Republicans missed the boat  to make changes to the census.

"The time to stand up was in April of 2007," Mikulski admonished. "Did you know that the census is mandated by law to submit the questionnaires to congress? And they did. 

"So for one year, from April 1, 2007, to the close of the review by congress one year later, April 2008, there was plenty of time to say, 'We don't like the questionnaire. We want to add a citizenship question,'" she added. "That was the time and the place."

Fox News' Trish Turner and The Associated Press contributed to this report.