WASHINGTON - Senate Democrats have blocked a GOP attempt to require next year's census forms to ask people whether they are a U.S. citizen.

The proposal by Louisiana Republican Sen. David Vitter was aimed at excluding immigrants from the population totals that are used to figure the number of congressional representatives for each state. 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 the data.

The plan fell after a 60-39 procedural vote made it ineligible for attachment to a bill funding the census.