How congressional Democrats could fight a Trump wall national emergency declaration

The  intent of the 1975 law was to allow Congress a block a presidential emergency by simple majority vote. A joint resolution by Congress to terminate an emergency can be vetoed by the president. McConnell said Trump could do exactly that.

'The president could win anyway by vetoing the bill and then trying to get enough votes to sustain it, so may ultimately be able to prevail on the national emergency alternative,' McConnell told Fox News on Tuesday. 

Still, some GOP senators are already expressing discomfort over such a vote, and have asked Trump this week – both publicly and privately – not to put them in that position.

'It's not my preferred choice,' said Sen. John Kennedy, R-La.

'I hope he doesn't do it,' said. Sen. John Cornyn, R-Texas.

'It might be a – you know – a tough vote to win here in the Senate,' said Sen. John Thune, R-S.D.