As a simple strategic proposition, Trump needs to pull the trigger on this. Now.
Trump has already admitted to obstruction of justice on national tv. Mueller is looking into his personal finances, which scares Trump to death. There's Flynn, Don Jr.... There is precisely one way to make sure that Trump never faces any criminal charges. Pardon himself (and his cronies) as his last act in office (since no charges can be brought against a sitting president, and no charges will be brought against his associates while he is in office anyway). There is a high likelihood that Trump will do so. This turns out better for him if he gets it over with now.
Keep in mind the basic pattern in presidential elections: two, then two. This is the motivating observation behind the Alan Abramowitz "Time for a Change" predictive model. You know, the one that called 2016 correctly? The one that's always right? Even if Trump gets two terms, he'll probably be followed by a Democrat, who will be under intense pressure to keep the heat on Trump, but the way Trump's approval ratings look now (an important factor in the Abramowitz model), he may not make it to two. A Democrat who beats him in 2020 on the basis of the Russia scandal won't do what Trump did with the "lock her up" thing because, unlike the email "scandal," this is real and major. Trump will, in that case, need to pardon himself before being succeeded. He won't be able to take the risk of facing charges. Clinton had already been cleared by the DoJ. Remember how Comey's handling of it was the original excuse for Trump to fire him? Yeah, that won't be the case. Trump will have to pardon himself, just to be safe.
If a pardon is forthcoming anyway, why wait? The effects of pardoning himself now are twofold: it stops Mueller dead in his tracks, and it creates another "scandal."
Let's start with Mueller. Mueller can be fired, but problems ensue if an investigation continues. There is always the risk that some lower-level schmuck accidentally does his job. On the other hand, if everyone involved has already been pardoned, then there is nothing to investigate. Trump really can shut down the investigation on the grounds that there is nothing to prosecute because everybody has been pardoned. That is the surest way to prevent Mueller from digging up whatever it is Trump doesn't want him to find.
At that point, the only route to an investigation would be Congress. A Republican Congress would never conduct a serious investigation, as we have seen, and even if the Democrats take Congress in the 2018 midterm-- even a single chamber would be enough to grant them subpoena powers-- all Trump would have to do is stonewall for two years, and that's it. Don't kid yourself. If Democrats take control of a chamber and start investigating, Trump won't hand over a single document. Ever. His people won't respond to a single subpoena. Ever. They will extend the claims of "executive privilege" beyond all historical precedent, and dare anyone to stop them. The only thing that could ever stop Trump is impeachment, and even if Democrats had the will, they'd never get the votes for it.
That moves us to the second fold-- the "scandal" of pardoning himself. Note the quotation marks. In legal terms, yes, Trump can pardon himself. The Constitution is a shitty document, and the framers fucked up on that one. By not prohibiting it, they left it open. The president can pardon himself, but doing so doesn't prevent impeachment, and like I've been saying, Congress won't impeach him. It would still be a breach of norms, and basically an admission of guilt, in the same way that taking the 5th seems like an admission of guilt. Now, in a trial, you can't convict based on the fact that someone took the 5th. That's kind of the point. But, it looks bad, and it would look bad for Trump to pardon himself.
But, then again, this is President Pussy-grabber, and he already directly admitted guilt to Lester Holt. He will insist that he is doing it because the scandal is fake news, liberal conspiracy, blah, blah, blah. And his entire party will stand with him to minimize the partisan damage. If you want public opinion to move dramatically, you need all of the messages to move in one direction, as we learned from John Zaller, which is why I've been arguing (see, for example, here) that Trump's support won't drop that far simply because his party elites won't abandon him. Ever. Not even if he pardons himself.
And here's the catch. If he pardons himself at the end of his term, then you've got a Marc Rich scenario. Bill Clinton took a lot of flack for pardoning Marc Rich-- a campaign donor-- as a lame duck right after the 2000 election, before Dubya was inaugurated. Even his own party wouldn't stand by him on that, and there would have been no reason to do so. The earlier Trump pardons himself, the more he boxes the GOP into supporting the action. And, the less that comes out before the pardon, the more they are locked into what they have already said-- that the scandal is nothing, parroting Trump's bullshit.
As long as the conflict over the pardon is partisan, then it's just another partisan squabble as far as the unwashed masses are concerned, and GOP elites will defend the action, making it seem like just more partisan bickering over a scandal that was, itself, also partisan bickering.
But, that doesn't work if Trump waits until after Mueller digs too far into Trump's finances and finds whatever it is Trump has been hiding. There is always the possibility that Mueller finds something so bad that it brings the whole thing down. Let's bring this into consideration. Yes, let's consider the possibility that Trump is a Russian puppet. He can't get financing from American banks because of his sleazy business practices, and Don Jr. has admitted to his dealings with the Russians. What if he really is so tied into the Russians that he just has to do whatever Putin says, and Mueller finds so much Russian debt, combined with more direct contacts between Trump's people and FSB agents directly referencing that debt that this whole thing comes down? There is a possibility of this going so far that the GOP can't maintain a unified front of support for Trump, and the way to prevent it from getting to that point is for Trump to pardon himself now.
The tradeoff is that, yes, this is ugly, and it could do at least a little damage to Trump's already low approval ratings. Trump's ideal outcome is for Mueller to be too incompetent to find whatever it is Trump doesn't want him to find, and to recommend no charges. But, that's a big gamble, and Trump's approval ratings have been hovering around and below 40% for a while at Gallup. It is hard to see, short of a war (which I have been warning about), what brings those up, which could put 2020 out of reach anyway. The smart move? Pull the trigger on those pardons. It is the most effective way for Trump to keep his secrets.
Of course, Trump has never been one to play it safe. Still, there is no way in hell he ever faces criminal charges for anything. If he thinks he will face charges, he will pardon himself, at least as his last act in office. That depends on what comes from Mueller. But, if it's going to happen anyway, he can reduce his pain a lot by just pardoning himself now. That way, he has more justification for shutting down the investigations.