I think your focus here is a little narrow given the broad scope of "the investigation". Let's take a look at what has been revealed thus far.
We have clear evidence that Donald Trump Jr. along with Jared Kushner and Paul Manafort met with a Russian attorney with the hopes of receiving damaging information on Hillary Clinton. The information never existed in the first place and the meeting ended after 20 minutes.
We have clear evidence that Democratic political operatives worked directly with the Ukrainian government in an effort to damage Donald Trump.
Ukrainian government officials tried to help Hillary Clinton and undermine Trump by publicly questioning his fitness for office. They also disseminated documents implicating a top Trump aide in corruption and suggested they were investigating the matter, only to back away after the election. And they helped Clinton’s allies research damaging information on Trump and his advisers, a Politico investigation found.
A Ukrainian-American operative who was consulting for the Democratic National Committee met with top officials in the Ukrainian Embassy in Washington in an effort to expose ties between Trump, top campaign aide Paul Manafort and Russia, according to people with direct knowledge of the situation.
Ukrainian efforts to sabotage Trump backfire - POLITICO
We also have clear evidence that the Golden Showergate dossier was put together by a firm hired to do opposition research for a Republican and then later was hired by a Democratic client for the same purpose.
In this case, the request for opposition research on Donald Trump came from one of his Republican opponents in the primary campaign. The research firm then hired one of its sub-contractors who it used regularly on all things Russian: a retired western European former counter-intelligence official, with a long history of dealing with the shadow world of Moscow’s spooks and siloviki (securocrats).
What we know – and what's true – about the Trump-Russia dossier
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By the time the contractor had started his research, however, the Republican primary was over. The original client had dropped out, but the firm that had hired him had found a new, Democratic client. This was not necessarily the Hillary Clinton campaign or the Democratic National Committee. Opposition research is frequently financed by wealthy individuals who have donated all they can and are looking for other ways to help.
https://www.theguardian.com/us-news/2017/jan/11/trump-russia-report-opposition-research-john-mccain
Of the three examples of obtaining or attempting to obtain information from a foreign source to damage a political opponent the one involving Donald Trump Jr is by far the least egregious.