David H. Stevens, the commissioner of the Federal Housing Administration who steered the agency through a critical stretch of the housing downturn, is expected to leave his post this spring, according to people familiar with the matter. Officials wouldn’t confirm or deny the pending departure. Mr. Stevens declined to comment. Mr. Stevens has played key roles shaping the Obama administration’s housing policies at the FHA, an agency that has occupied a vital role in healing housing markets by continuing to make low-down-payment mortgages available. He took the helm of the government loan insurer in July 2009 at a time that it faced rapidly rising losses from mortgage defaults and dwindling reserves, raising the prospect of a taxpayer rescue.