Much has been written about how Trump's popularity inched up from a low of 37% (according to the Real Clear Politics (RCP) poll average) in December 2017 to 43% in June of 2018. It has stubbornly remained around 43% ever since as can be seen in the graph above. His disapproval ratings have fluctuated between 52% and 54% over the same period.
Much less reported is Trump's approval ratings on the economy. The RCP average of polls on this question is 50.8%, a slight majority. The polls used to create this average over the last two months can be seen in the image above. The approval ratings of these polls range from 49% to 55%. The letter RV next to the sample size for the poll means that they limited their sample to registered voters. The A next to the sample size means that all Americans were included in the sample. There is no graph showing how this rating has changed over time but the few times I have looked at this average has been consistent. His disapproval ratings on the economy average to 42% and range from 36% to 47%. There aren't as many polls on this question as there are on his overall popularity.
One poll that is absent from the above table in Rasmussen Reports. They come out almost daily with overall approval ratings for Trump ranging from 46% to 50%. They restrict their sample to likely voters (the only ones in the RCP average to do so) and their estimates are consistently the most generous to Trump.
There are even fewer polls asking about Trump's approval on Foreign Policy. Not surprisingly the RCP average on this is lower than his overall approval rating and his approval rating on the economy at 40.7%. Rasmussen's polls are not on this question either.
For the generic congressional race polls the Democrats have a 6.8% lead in the RCP average. Only Rasmussen limits their sample to likely voters the rest use registered voters on this question. These numbers have been more volatile than the approval ratings for Trump. Gerrymandering in many states gives the GOP an advantage in states where the two parties have an equal number of voters. Pennsylvania just had it's congressional districts redrawn and it remains to be seen what impact it will have.
The Republicans running this year probably will stress the economy while Democrats should be stressing Trump's foreign policy as his approval ratings are weaker there. This doesn't mean that Democrats should ignore domestic/economic issues such as health care, immigration, climate change, and income inequality. Foreign policy provides a fuller picture.