The signs look pretty good for Democrats. For one thing, the president's party usually loses seats in the midterm elections. The only time this didn't happen in recent decades is when Republicans won seats in the 2002 election but in that case they were able to use 911 to their benefit. This just Oracle trend is worsened by the fact that Trump is a historically unpopular president. Even presidents who are overall popular tend to lose seats and unpopular presidents tend to lose a lot of them. The generic ballot also benefits Democrats this is a poll question which asks whether people would vote for a Republican or Democrat without any other kind of information about the candidate. Democrats are currently winning this poll by about 12 points. In 2010, when Republicans 160 seats and retook the house, they had about an eight or nine point advantage on the generic ballot. The 2017 races also seem to bode well for Democrats. They 1 an awful lot of state-level special elections in 2017 as well as, famously, the Alabama Senate race. They also swept The Virginian races with their gubernatorial candidate, generally regarded as a fairly uninspiring figure, winning by 10 points, one of the largest margins in modern Virginia history. Democrats in the Virginia state house also wiped out a republican supermajority turning it into a near tie. Even in the special elections which they lost, including four out of five congressional special elections, Democrats did much better than past performance suggested they would. For example, in Kansas, the Democratic candidate, with little name recognition or money, improved 20 points on the performance of the 2016 Democratic candidate. These elections suggest an awful lot of energy on the Democratic side, and energy tends to win midterm elections.
I think there's a decent chance that Democrats retake the house. They need to win about 25 seats in order to do so. Coincidentally, there are about 25 Republicans who represent districts which Hillary Clinton won. These people will be top targets for Democrats in 2018. The Democrats are also helped by a wave of retirements among Republican legislators. Iirc, about 40 Republicans have announced their retirement. This helps Democrats because incumbents are generally thought to have an advantage in elections. An incumbent Republican my also be better able to defy an anti-republican Wave by pointing to their individual accomplishments. In the Senate, the situation will be harder for the Democrats because of the map. Democrats only need to flip two seats in order to retake the Senate. However, only about 10 seats up for election are held by Republicans and many of those are in deep red states that Democrats likely do not have a chance of winning. If Democrats are going to retake the Senate then the most likely path will be by flipping Arizona and Nevada. Both of these states have large Hispanic populations and either lean Democratic, in Nevada's case, or are trending Democratic, in the case of Arizona. However, this plan requires that Democrats also hold on to all of their other Senate seats. The problem is that there are about 10 Democrats who represent states which Donald Trump won in 2016. A number of these candidates will likely be easily reelected. However, there are some, such as North Dakota Senator Heidi heitkamp or West Virginia senator Joe manchin who could very well lose because they are running in states which heavily favored Trump. If any of them do lose then Democrats will have to pick up another seat someplace else and it's difficult to see where that could happen unless Congressman Beto O'Rourke pulls out an upset win over Ted Cruz in Texas.