Trump’s order to start building the wall is not by-passing Congress. Back in 2006, Congress passed a law authorizing the construction of 700 miles of fencing along the southern border, as well as additional lights, cameras and sensors to enhance security.
Then, Democrats got control of Congress a few months later and put an amendment into a 2008 spending bill that took all the teeth out of the 2006 law.
IOW, Democrats buried an unpopular measure they wanted inside a spending bill so they could avoid standing up and taking responsibility for blocking something Americans want done.
But, and this is a big but, the original law was never repealed. Congress doesn’t have to pass a new law to begin construction. All they have to do is package the funds necessary to build the original and substantial law into a massive spending bill.
Democrats would have a politically hard time blocking construction, since it would require them to block the spending bill and shut down the government, something they mocked Republicans for doing in the past.
Barack Obama, Joe Biden and Hillary Clinton all voted for the 2006 law. So did Chuck Schumer and a whole host of other Democrats in the House and Senate, many of whom are still in Congress.
President Trump has said the wall will be from 35 to 50 feet high and made of pre-cast concrete. He also plans to figure out a way to make Mexico pay for it.