At the AEI blog, Jim Pethokoukis offers some ideas for "how Republicans can win even if the economy keeps improving."
The main suggestion is to focus on the ways in which things are better, but not good enough.
And yes, that's probably the only alternative. But, warning, the nimblest of all American politicians, Franklin Roosevelt, bequeathed to his successors a pre-emptive answer to that line of attack, in his speech to the 1936 New York Democratic state convention:
In the summer of 1933, a nice old gentleman wearing a silk hat fell off the end of a pier. He was unable to swim. A friend ran down the pier, dived overboard and pulled him out; but the silk hat floated off with the tide. After the old gentleman had been revived, he was effusive in his thanks. He praised his friend for saving his life. Today, three years later, the old gentleman is berating his friend because the silk hat was lost.