A Republican lawmaker wants Fannie Mae and Freddie Mac to buy portable mortgages to open up much-needed housing supply. The congressman said he heard from residents who are experiencing the so-called lock-in effect, who are ready to move but unwilling to ditch their current mortgage rate. The average 30-year fixed rate mortgage closed July at 6.81% , its highest mark in over a year. Portable loans, in which the loan moves with the borrower, are different from assumable mortgages, in which the loan stays with the property. The GSEs restrict assumable loans for fixed-rate mortgages to certain circumstances like death and divorce.