why not? Campaign against Pelosi and the Democrats and their policies, compare and contrast what their policies did the last time they were in charge to what we now have with them not in charge. It's a simple and effective message. Let's get with the program, folks. If an opponent makes an issue of President Trump, the retort is "really? We should enact terrible policies because you don't like the President's personality? What kind of insanity is that?"
the left’s base is on fire because they’re losing their free ride.
our side is doing well with the economy and might get complacent.
I agree with that. The undecided voter has to be convinced that if Pelosi becomes speaker of the House the economy will tank. I don't even believe that. I believe if Hillary had won the economy would still be stuck in neutral, but that's irrelevant at this point. At least in districts carried by Trump, the focus should be on the border and impeachment. Force the Democrat opponent to respond on those things. Ann Coulter continually says the border should be the focus. Talk about the loony ideas coming out of the Dem party too, like abolishing ICE.
Making the economy the primary focus of your campaign is a loser. The Democrats captured the Senate back in 1986, right in the middle of an economic boom, when Reagan was President. The losing R's were probably counting on the economy to carry them through, while the Dems had their usual attacks: "the GOP is anti-women, anti-workers, anti-environment, anti-education blah blah blah" just like today (except today the Dems have gone much further left).
To counter attacks you have to have attacks of your own that differentiate you from your opponent. There's no way to make the strong economy into an attack on the Democrats that makes any sense to the typical undecided voter. It's more like playing defense. You lose playing defense.