House Democrats are laying the groundwork for another major immigration debate later this year, despite the risk that it could prove politically destructive for their party. Moving broad legislation that would put millions of illegal immigrants on a path to citizenship seemed politically impossible until fairly recently and may still end up too hot for Congress to touch. Yet Obama sent a signal this month to Hispanic Democrats that he is still committed to the cause and that he plans to host a White House immigration summit before the end of May. That has bolstered the hopes of the legislation’s...