Census data say 37% of homeowners with mortgages are spending 30% or more of their before-tax income on housing -- the threshold where the government says a home becomes unaffordable. That's up from 27% in 2000.

For some, the financial burden is even worse: 14% of homeowners with mortgages -- more than 7 million households -- spend at least half their gross monthly income on their home loan, property taxes, insurance and utilities, up from 10% in 2000.

Rachel Drew, research analyst at the Joint Center for Housing Studies at Harvard University, was most surprised by a sharp rise in the number of financially strained homeowners from 2005 to 2006.

In that one year, more than 1.5 million additional homeowners began spending 30% or more of their income on housing, including 680,000 who are spending more than half.

Drew told USA Today that the number of financially squeezed homeowners will to continue to rise as 2 million adjustable-rate loans reset this year and next, adding hundreds of dollars to monthly payments.

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