Perhaps the American dream of homeownership is not for everyone.

That may sound at odds with a bedrock notion of society promoted by presidents for decades. But many experts say it is a message that can be drawn from the rising troubles with mortgages provided to home buyers with weak credit.

Several large mortgage companies have stopped making new loans, and others have tightened lending standards.

Hundreds of thousands of families who bought houses in the last two years — using loans with low teaser interest rates and no down payments — are now losing them.

Their short tenure as homeowners calls into question whether the nation’s long drive to increase homeownership — pushed by both public policy and financial innovations — has overstepped some boundary of demographic and economic sense.