Why did president roosevelt feel the need to propose expanding the number of justice on the supreme court?

The Supreme Court had been striking down New Deal Programs as unconstitutional. Before Roosevelt introduced this plan, the Supreme Court have rejected many elements of his New Deal plan because the authority held by the Executive was too much and considered unconstitutional.  He thought that by adding more justices to the Supreme Court, it would protect his legislation.  Still it was rejected by both liberals and conservatives.