Dean Baker, President Obama Comes Out for Subsidizing Mortgage Backed Securities:

… government backing is not necessary for 30-year mortgages, as is shown by the existence of the 30-year jumbo mortgages which are too large to be eligible for government guarantees. The interest rate on these mortgages is typically 0.25-0.50 percentage points higher than the interest rate on conforming loans that can be purchased by Fannie Mae and Freddie Mac.

So the story here is not really about the existence of 30-year mortgages, but rather the price. The program being pushed by President Obama effectively subsdizes mortgage interest rates by subsidizing mortgage backed securities. If the goal to make homeownership more affordable for moderate income people, this is an extremely inefficient way of doing so.

Under the Obama administration’s proposal the vast majority of the subsidy would go to higher income homeowners since there will be a bigger subsidy for people who take out bigger mortgages.