Dems are simply trying to keep some sort of "controversy" going until the election so they can focus on that drama instead of the non-existent platform they aren't running on.
It's hard to avoid controversy with Trump. He seems to be a walking talking controversy machine. It would have been simple for Trump to call the Ukrainian president and congratulate him on his win. He could even have talked about the past corruption and the hope for continued work in striving to reduce it more. But Chaos, even a low level management coffee fetcher would know it would be inappropriate for him to ask that an investigation be opened up on his political rival.
I know this fact doesn't matter in the narrative you are presenting but it wasn't the Democrats that filed a Whistleblower claim through the proper channels following proper protocol. That it was done and came to light leaves them little choice than to launch an inquiry unless... Unless we are at that point where it really doesn't matter what Trump does. I realize about 40% of registered Republicans are already there, but they don't represent everyone. Some people still care what our President does