For Long Island, the stakes in this year's Census count couldn't be higher. An undercount could mean millions less in aid, money the region can ill afford not to receive. What's more, it could mean lost political clout, lost jobs and lost opportunities.

"The cost of an undercount would be catastrophic," said Seth Foreman, head planner for the Long Island Regional Planning Council.

An undercount in 2000 - in a region that has an unusually high Census response - cost Nassau and Suffolk millions of dollars in lost federal aid. Long Island cannot afford for history to repeat itself.

This time around, with property taxes at historically high rates and families squeezed by the recession, Long Island will need to lay claim to every penny of federal and state aid it is entitled to.

According to planning council estimates, Long Island could lose $1,611 in federal aid for every Long Islander not counted. Each year, the federal government distributes about $300 million in federal aid nationwide based on "formula grants" anchored to Census data.

That money gets passed along to New York State - and on down to counties, towns, cities and villages - in some 70 different programs, from Med-icaid to drug-prevention programs to wildlife preservation.

In New York State, formula grants also are used to determine local school aid and social services funding. If Long Island can't claim its $1,611 per person in federal money, these funds will go to another municipality that can.

As it is, according to the planning council, Long Island is being shortchanged by $6.6 billion from Washington and $3.2 billion from Albany. Those amounts come into play because the region pays out proportionately more taxes to the state and federal governments than it gets in aid in return.

But the region won't be able to make the case for more equity if there's an undercount.

We can't afford that.

We also can't afford to lose representation, in Washington or Albany. As a result of the 2000 Census, New York fell from second to third most populous state in the nation. With an undercount in 2010, it could fall to fourth, behind Florida.

In the 1970s, New York State was populous enough to send 50 representatives to Washington. Officials predict that the number could fall to 27 with the 2010 Census. Closer to home, an undercount of 200,000 would translate into Long Island losing one seat in the state Assembly.

That would spell disaster for suburban Nassau and Suffolk in a state where the center of political power rests in New York City.

And then there's the question of how wealthy Long Islanders are. It's not unusual for a region to lose aid - driven by income and population formulas - because a large percentage of the population have high incomes.

According to Foreman, per capita income rises when population counts decrease, making an undercounted region appear to be wealthier than it really is.

Which is just one more reason why every Long Islander must count.

And that includes the estimated 80,000 undocumented immigrants from the Queens border to Montauk. For the record: Immigration status means nothing to the Census, or to population-driven formulas that determine federal and state aid.

"You have a situation where people are using services that Long Island should be reimbursed for," Foreman said.

To the estimated 200,000 Long Islanders living in the estimated 100,000 illegal accessory apartments in single-family homes in every community, here's a message just for you:

Get counted. Just because a Census form isn't coming directly to your basement or garage apartment doesn't mean you're in the clear.

The Census Bureau is planning to open "Get Counted" offices in libraries and other local venues soon. Get there. Get the Census form.

And for everybody else, fill the form out when it comes and send it right back.

We need that $1,611.