In 2011 the Justice Department made a decision that the American Wire Act only prohibited online sports gambling and did not cover online slot machines or other online parlor games. This decision largely allowed individual states to decide how to regulate online gambling within their own borders and allowed states like Delaware to quickly jump into the game.
However, during his confirmation, Attorney General Jeff Sessions said he was shocked by the 2011 decision and did not necessarily agree with it. He also stated it was a decision he was likely to revisit once becoming Attorney General, which he now is.
The story gets even more murky ( welcome to politics) since Sheldon Adelson, the Las Vegas Casino owner reportedly donated $5M to the Trump Inauguration Fund, along with $11M to the Trump campaign and another $20M to a Republican senate PAC. One of Adelson´s main issues is the rise of online gambling, so you can imagine a lot of influence was bought with that $36M in campaign funding over the last 12 months.
An act by the Department of Justice might be necessary since very little progress has been made in either the senate or house in regards to restricting online gambling with Republicans believing it is best left as a state issue. President Trump has also stated in the past that online gambling is unstoppable, so either the USA takes a piece of the pie, or we lose out to other countries who allow it. However, as with many of the things said by Trump during the campaign, it does not seem impossible that he will allow his Attorney General to reverse the 2011 DOJ ruling.