To put Trump’s polling numbers in perspective, Ronald Reagan‘s approval rating in the middle of 1983 according to Gallup was 45%. In 1984, his approval rating was 55%. In other words, when the American people actually sat down and listened to the accomplishments that Reagan had made during the presidential campaign, his approval ratings jumped substantially. Most Americans are on vacation and are not paying attention to politics at this time. When they focus next year on Trump and his accomplishments, his approval rating will jump, and he will be reelected easily.
We can't use the Reagan situation in 1983 as a template for this. We were still in the early phase of recovering from a very deep recession at that point. By the following year the very powerful recovery was apparent to most.
The Trump campaign needs to run lots of ads during the primary season making the case that the economy's strength is due to changes in policies under Trump - and that the economy was lousy under Obama. The Dems have put out this fairy tale that the economy was strong when Obama left office and they repeat it constantly. That has to be directly challenged and destroyed. Just repeating how good the economy is won't work.
They should run ads building up Pres. Trump on other accomplishments, like energy production, rebuilding defense etc. during the primaries.
IOW, don't wait till the Dems have a nominee and then start attacking him/her. They need to build up Trump's numbers first. The primary season is the perfect time to do that.