Finally specifically regarding the ST Louis from sterling, unitedstateshistory
“President Roosevelt could have issued an executive order to admit the St. Louis refugees, but this general hostility to immigrants, the gains of isolationist Republicans in the Congressional elections of 1938, and Roosevelt’s consideration of running for an unprecedented third term as president were among the political considerations that militated against taking this extraordinary step in an unpopular cause”
So FDR could have issued an executive order based in humanitarian concerns and for political reasons chose not to.