Several Massachusetts newspapers are reporting that independent Seth Moulton could conceivably be elected to the U.S. House this year. Although the Democratic primary is not until September 6, it is too late for Moulton to get on the Democratic primary ballot. If he is going to be an independent, he needs 2,000 valid signatures by July 31. See this story and this story.
It is certain that the incumbent Democrat, John F. Tierney, will be the Democratic nominee. He is the only person who is on the Democratic primary ballot for that seat. However, as the news stories explain, he has lost support recently.
The ability of an independent candidate to get on the November ballot as late as July 31, or even later, is one of the advantages of an election system that does not use “top-two open primary” rules. If Massachusetts had a top-two open primary, it would be too late for anyone to enter the race this late in the election year.