If he had lost the two Houses, he still probably would have found a way to claim for a victory. He does however have a point: Republicans gained seats in Senate which gives him the power to nominate and confirm judges of the Supreme Court and other Officials of the United States. Moreover, there was not as big as a blue wave compared to the polls’ predictions so, to that extent, it is a little victory for President Trump.
Democrats will have to play it safely with the candidate they choose for the 2020 Presidential elections. If someone extremely left liberal such as Bernie Sanders is chosen, they will likely lose out on a lot of moderate Republicans who would have difficulty backing someone very left wing vs. Trump if he runs for a second mandate. Of course, two years is a political eternity! New personalities could arise; there could be someone who really stands out of the new Congress, exerts a great power in a committee chair, digs in investigations...
To what extent did the midterms bring a wind of change in US politics?
The female push into politics was the real highlight of those midterm elections. The record number of women elected into Congress in both parties will hopefully drive for compromise and help bipartisan agreements. Furthermore, the great number of minority candidates running for the first time underlines the real desire of expression coming from oppressed communities. The youngest female candidate, the first Native American candidate and the first Muslim candidates were elected in the House of Representatives. This year will be critical in terms of keeping the energy alive and driver for change in US politics.
Although we should avoid using the word "referendum" because it gives too much importance and power to Donald Trump’s influence on the ballot, the exceptionally high turnout definitely showed the energization of the voter base - and this was largely a direct consequence of Trump himself. At the end of the day, the midterm elections turned out to be around the question of whether or not the candidates agreed with President Trump.
Overall, the results of the midterm elections conformed more or less to expectations. The split Congress underlines more divisions and polarization in the US political system and reduces incentives for people to reach across the aisle. A Republican Senate also means Trump will have an easier time getting his judicial nominees confirmed, and those picks are likely to drive the discussion for decades to come on divisive culture and identity issues.
There is little doubt that the Democrats' voter base will push to go after Mr. Trump for investigations and maybe impeachment but the Republican Senate, indebted to Trump, still remains decisive, because it is the sole power to conduct impeachment trials. This of course balances the chances of remarkable changes in the next two years.