Stephen Coonts latest book “Liberty’s Last Stand” is all about this. President Barry Soetoro takes a black swan terrorist event to declare marshal law and dismisses both congress and the courts until resolved.
The book takes place Aug 2016 and, while entertaining, is a long stretch. TX does declare independence though so that’s a good thing.
If things had progressed as far as they did in the book society would collapse - no electricity when barry shuts the grids down would cause immediate anarchy and doubtful things could recover from there.
Any way, it is possible if not plausible.
Coups have happened in other countries before, with mixed results; those staging them had best make sure their loved ones are secure. The Spanish Civil War dragged on a few years because people simply stormed the police stations and barracks (in areas where socialist/communist sympathies were strongest) and armed themselves; if the police/military resisted they were killed outright (including some intercepted before they could reach the stations/barracks).
TX has its own power grid, not connected to any other grids in the US - nuff said.