This simply isn't true. In fact, Trump has given them a lot of stuff to campaign on. The specific campaign issues vary from race to race but they include things such as family separation and immigration policy, the environment, the economy, tax cuts, racism, health care, and Corruption. The Democrats have such a plethora of campaign issues that they can really pick and choose which ones they want to engage with.
In contrast, the Republicans seem to be the ones without much, if anything, to campaign on. Republicans have tried to campaign on the economy, but that hasn't seemed to work. They're single accomplishments so far was the tax cut and hats unpopular among most Americans. That's about all they have to campaign on. What they are left with our culture War issues, which have not worked well in swing state districts, and attempting to Brand every candidate as a tool of Nancy Pelosi