Actually Mueller will have to either wrap it up or go into a “dark period” before the 2018 midterms. At least, that’s what he’s supposed to do per DOJ. But he has a very leaky ship, and if he takes his cues from his buddy James Comey election DOJ Guidelines for special investigations will be discarded.
https://www.wsj.com/articles/mueller-probe-might-have-go-dark-for-midterm-election-1525604400
That's obviously a concern to Mueller, however there is a critical difference between the impact of the current investigation on the 2018 Congressional election and that of the Hillary investigation during the 2016 election. Trump nor any of the main characters in the Mueller investigation won't be running for office in 2018!
Because Hillary was running for president
at the time of the investigation .. . when Comey, mistakenly as it turned out, thought the investigation of Hillary's emails was complete he felt an obligation to announce the findings before the election. If the FBI was not going to recommend prosecution to DOJ then he believed the country should know that. I think he made the right decision at the time given the information he had. Then when Wieners lap top turned up, the investigation had to be reopened. Now he looks like a fool to have spoken too soon, but only if you don't place yourself in his shoes. Then you may see it differently.
By the time Trump runs again he'll have been proven pure as the driven snow and the Mueller investigation will have resulted in absolutely no harm to his 2020 campaign. Mueller will have been run out of town on rails having been revealed as a hopeless lackey of the 'Deep State', and Trump will have been re-elected in a landslide! (cough, cough).
P.S. You may not have noticed, but Mueller has been 'in a dark period' throughout this investigation. There is no possibility that is going to change. We won't, probably in our lifetimes, know the full panoply of information he has turned up. But we'll eventually learn of the relevant bits when his report is issued to the DOJ and to Congress... Little morsels have emerged as they have been parts of public indictments. For example, Manafort's Tax Evasion, the specific names of Russian Hackers, etc.