From the moment President Trump began speaking Tuesday night, in an Oval Office address urging the public to support his controversial border wall, Democrats who may run for the White House in 2020 were responding in real time with sharp criticisms and common themes:

• Mr. Trump is holding federal workers “hostage” — a popular word in the responses — and causing them economic hardship.

• A wall would be ineffective and “medieval,” another popular word.

• The president’s willingness to shut down the government over his demand for a wall has actually harmed border security by denying pay to Customs and Border Protection officials.

• Mr. Trump, not congressional Democrats, is responsible for the shutdown. The Democrats repeatedly noted that the Senate unanimously passed a bill last month that would have kept the government open.

• There is no immigration crisis at the border, and any crisis that does exist is a humanitarian one caused by the president himself.