What stops the Republicans from taking judicial action against Obama? The first responsibility of a government is safeguarding it's citizens. He's willingly, knowingly and actively exposing them to the threat of terrorism.
The republicans have two options. One, they can block funding for things he wants. They promised voters they would do that, then immediately folded. This in large part explains the fervor for Trump, who is seen as an outsider who will ignore or defy the party bosses. The other option is impeachment, which the republicans are far too timid to try. It would be pointless anyway, as democrats in the Senate could block it.