A federal judge said this week that the Obama administration is likely violating the law by telling immigration agents and officers not to arrest illegal immigrants they deem low priority, in a case that could upend President Obama’s enforcement policy. For the last several years the administration has said it will no longer arrest most illegal immigrants, arguing it wants to focus only on those with serious criminal records or gang ties. The Homeland Security department said it was using “prosecutorial discretion.” Immigration agents and officers sued, saying federal law requires them to make the arrests, but the Obama administration...