It can be legally considered a form of opposition research, which would constitute a violation of campaign finance laws when solicited from a foreign government. It's pretty similar to the Russia situation actually, just the opposite as this is the solicitation rather than the interference itself. On the broader scale, it would be seen as an abuse of power which itself would be the focus of impeachment. Impeachment doesn't require a specific law to be broken, so the strict legal specifics won't matter as much in the end.
The democrats+Ukraine issue was just simply asking them to comply with existing requests of cooperation in an existing investigation. That's very different than what's happening here. There was nothing in the letter about deteriorating relations with the democrats or suggesting any kind of consequence. Part of the letter was actually them being concerned Ukraine was failing to cooperate because they wanted favorable relations with Trump.
As far as the timeline of the Ukraine aid goes, I haven't heard that, but there were meetings after this call took place, and of course the possible suggestions of it within this call (again, we don't know the real transcript yet). I mean, I'm all for legitimate investigations of corruption in any form. But no one honestly believes Trump suddenly decided to cut off aid right before suddenly deciding to ask for dirt on the democratic frontrunner out of good intentions. The democratic house probably won't believe that, at least.