Bank fraud and campaign finance irregularities are what they're pursuing on Cohen (apparently, based on the documents they asked for in the warrant).
Those are the issues they're more or less forced to investigate, due pretty much entirely to public pressure (and where do you suppose that is coming from?)
All of this is about affecting public opinion. The reality is that if you looked closely enough at pretty much any lawyer or staff member connected to any political campaign, you could find sufficient evidence to justify such a warrant. The difference between who gets raided and who isn't is pretty much purely driven by who's name is being pushed in front of the public by the media, and thus who's being demanded to be investigated.
That's not the end goal. They're looking to flip Cohen, and if they do, he knows nearly everything that Trump knows.
Which wont happen. They know it wont happen. This is about the same thing all of the other warrants, testimony, allegations and whatnot have been about: Feeding public perception (or deriving from it). The fact that every single time something like this happens, the immediate assumption in the public perception is "this is important! It means he'll flip on Trump and then we'll know the secrets of <whatever>", is your first clue that this perception is exactly the purpose.
Including, yes, about the Russians. He was the bag man and allegedly the negotiator for at least some of the alleged foreign-money type campaign shenanigans.
There will be plenty of things that will appear to be "foreign-money type campaign shenanigans", for the simple reason that anyone who does any business with foreign companies will have documents that can be presented to the public as though they are somehow "shenanigans". OMG! He did... business... with the Russians! It must be significant! Let's all talk about how significant this must be and speculate about how this could be something wrong, illegal, or whatever.
But, you know, we'll see. This may be Watergate, or then again it may be Whitewater.
Legally? Neither. In the media? Both! Or worse! Whatever. At the end of the day, the real problem here is that you have an investigation that has nothing to do with pursuing a violation of a written law, and everything to do about digging up speculation about a politician. I'm still waiting for someone to even come up with the actual legal statute that they think Trump may have violated here. Hint: There isn't one. There's no law called "Russian Collusion". Not unless you get into areas of passing secrets to a foreign government, which even the most absurd conspiracy theories aren't suggesting.
You guys actually think there weren't foreign governments who wanted Obama to be elected instead of a GOP alternative? You don't think those governments funded issues based advertisement in favor of positions Obama held? You don't think that every candidate for president has supporters and detractors around the world who engage in this sort of thing? You're naive if you think that. Of course foreign governments have an interest in our presidential elections because who sits in that office affects foreign policy of the US, which affects them. Of course they fund organizations inside the US who in turn advocate for positions and issues connected to those politicians, just like anyone else can and does.
This is not illegal. It's never been illegal. It is, at best, a political issue to argue that "so and so is liked by country A, and we don't like country A, so by association we should not like so and so". Which, um, has been tried, but is a hard sell to make in this case, and especially in this election, where all of the evidence shows that the Russians assumed that Clinton would win, and put all of their efforts into putting themselves into the best position they could when she did.
The Russians were not trying to help Trump win the freaking election people. The Russians were trying to weaken Clinton as a president by running with the whole "stolen election" bit. That's what the leaked emails were about. At the risk of repeating a previously made point, if you actually bother to read the entire intelligence report on "Russian meddling", it details this. Because of this, there's more or less zero possibility of any evidence of "Russian Collusion" between Trump and the Russians. And certainly nothing that comes remotely close to anything illegal. Because nothing the Russians were doing required any interaction with the Trump campaign. They would have no reason to do this. Any more than they'd have reason to interact with the Sanders campaign in order to push the idea that Clinton and the DNC "fixed" the primaries. It's the same thing.
So what we'll keep getting is more and more nibbling around the edges, all the while ignoring that the supposed primary point of the investigation is complete nonsense. At this point, it's an investigation for the purpose of having an investigation, and it'll continue as long as there's political value to being able to put "under investigation" as part of the byline whenever speaking about Trump, the White House, and anyone close to him.
But hey. Let's just keep speculating that "this time", it'll be meaningful. It wont.