The timeline gets away from me the longer this goes on, but the earlier it can be shown that some elements of the prior administration was gathering dirt on Trump the more outrageous this becomes. They can't wash it away by saying they were legitimately investigating him in response to a Russian tampering threat that they didn't yet know about. Also, if it want as far back as the Republican primaries, then who the hell else were they into? ...And if they only focused on Trump, then why -- when everyone thought his run was going nowhere?
Or on the other hand, this could turn out to only be new news because we are finding out about it. It could very well be that sitting administrations spying on top candidates is common practice going back through every Presidential election in the recent past.