WASHINGTON — Senate Democrats on Thursday blocked an attempt created by Sen. Bob Bennett, R-Utah, to require next year's census forms to ask people whether they are U.S. citizens.

The amendment, sponsored by Sen. David Vitter, R-La., and co-sponsored by Bennett, to a bill that funds the census was aimed at excluding noncitizens from the population totals that are used to figure the number of congressional representatives for each state.

Critics said the 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.

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

"The current plan is to reapportion House seats using that overall number, citizens and noncitizens," 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."

Bennett first raised the issue earlier this year when he introduced a separate bill to identify and count illegal aliens in the census. He did that as several conservatives are challenging him in next year's election and contending he is soft on immigration issues.

If Vitter and Bennett had been successful — and if noncitizens 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 noncitizens were excluded, Louisiana and eight other states would keep or gain congressional seats that would go to California, Texas, Illinois and New York. Vitter said the other eight states are: North Carolina, South Carolina, Oregon, Pennsylvania, Mississippi, Michigan, Iowa and Indiana.

Vitter's amendment, however, would not have changed the way the congressional seats are allocated by counting citizens and noncitizens alike.

Census Director Robert Groves opposes the proposal and recently told lawmakers that it would greatly delay the decennial count.

Critics also said Vitter's plan would discourage immigrants from responding to the census and would be hugely expensive.