In electoral terms, that's another matter. 2018 is a midterm election. One of the regularities in politics is that the president's party loses seats in a midterm election. There are exceptions. In 2002, after 9/11, the Republican Party gained seats. In 1998, amid an impeachment that the public didn't think was appropriate, the Democratic Party gained seats. The Democrats are not pushing an impeachment (despite, well...), and we haven't had a big terrorist attack in a long time. When it happens, though, there can be a "rally round the flag" effect, in which the president gains public approval, and that can nullify the midterm election effect, as it did in 2002.
Midterm effects are most consistent in the House of Representatives. All 435 seats are up every year. The Senate is trickier because only a third of the Senate is up in any given year. This year, the same seats are up that came up in 2012 and 2006. Think back...
That means Democrats are defending seats that they won in strong Democratic years. Picking up more seats in a landscape like that is actually quite hard. So, it may look like the Democrats are really close to a majority in the Senate. At 49 seats, all they need is two more, but that's the wrong way to look at it. If all 100 seats were up, then sure. That would be the right way to look at it, but that's not how the Senate works. With the 2018 map, the Democrats don't have a lot of potential pick-ups.
That's why Roy Moore's loss is a bigger deal in electoral terms. He puts a Senate majority for the Democrats into the realm of plausibility. And that matters.
With control of the Senate from 2019-2020, the Democrats could do a lot.
1) Block any Trump appointment, including Supreme Court appointments. It looks like Kennedy is thinking about retiring. As I wrote a while back, Kennedy would be smart to retire soon. Like, now. That would make him far smarter than that idiot, Ginsburg.
2) Begin real oversight hearings and investigations. Mueller may get shut down soon, but even if he does, if Democrats get control of the Senate, they can start their own investigations, with subpoena powers, and everything.
3) Bargaining leverage over the budget. A lot of what the Republicans are doing is budgetary-- taxes and spending. Give the Democrats control of a chamber of Congress, and that stuff becomes negotiable because spending runs fiscal year to fiscal year.
4) When Trump gets nervous and angry, he self-destructs. Put Democrats in control of a chamber of Congress, and... we don't know what happens on the administrative side, so I'm just throwing this in here. Trump gets even more reckless and aggressive in some way because for all of his "I'm a dealmaker" bullshit, he's the worst negotiator ever.
Even with Doug Jones, the 2018 landscape is still hard for the Democrats. It is now within reach, though.
And here's where things get even messier, electorally speaking. Do Republicans finally start Huelskamping their wackos on a regular basis? They really, really need to. They lost a Senate seat in fucking Alabama.