Besides taking progressive gambles like including free college, alternative energy, single-payer, in your message, here's a radical idea from yours truly. Tell the truth about what tax cuts do to the budget.
Republicans are more fiscally irresponsible than Democrats because they put the countries spending on your credit card because they don't want to fund the government with tax revenue. Republicans hammer on balanced budgets and fiscal responsibility. But, the tax cuts they advocate for routinely lead to revenues underperforming, often hitting as low as 15% of GDP. Tie Republicans to the national debt, by pointing out they turn spending into public debt with tax cuts. If Democrats can play it right, they've been given two electoral nuggets of gold, supply side's failure in Kansas and GWB's incompetence.
Can Democrats make them into the party of fiscal irresponsibility? Because Republicans want to push tax cuts that decimate budgets. They are the cut and spend party. All you have to do is point to the 20 Trillion in debt, and blame it on Bush and Republicans irresponsible cut and spend.
Run a campaign ad that tells them Clinton left Bush a surplus and Bush turned that surplus into the highest % increase of deficit spending of all times. Because he cut taxes and started 2 wars. Then point to real GDP growth peaking at 3.6% before the financial crash, tie Republicans to the deficits and the financial crash, and then say, Trump wants to repeat this and throw in an anecdote Trump running a casino into the ground in Atlantic City.
I want to be clear that I think Democrats running towards the center, and appealing to Republican values as a Blue Dog Democrat does, is a poor strategy. People much prefer strong progressives to weak Democrats. If you'd like evidence of that, look at the 1,000 seats lost. I don't think the country is actually any more right than it was in 2009. I think if you look at the polling the country holds center left positions on marijuana legalization, single-payer, ending the wars, government corruption and money in politics, alternative energy. The country is center-left.
So, here's what I would do, tie Republicans to incompetent tax cuts that lead to budget crisis and say that Trump wants to repeat failed supply side policy.