Looks like wild violets. They are ferocious and spread like mad if you don't get them under control. The herbicide Triclopyr will work but might take several applications. If it's a small area Round Up will be quicker but will also kill the good grass too but since you're already planning to overseed that could be an option.

If that is everywhere and you don't like it, the only alternative is renovation with enough RoundUp applications to kill it out. I guess you could hand pull it. I've read one report of success doing that but it was a very lengthy process. They started in one corner and worked out for several feet. Then a couple days later they started again in the same corner to get the stuff they missed the first time. Of course that went much faster but it was necessary. Then a week later they started again in that same corner to get the stuff they missed the first two times. Ad nausium. They had a dirt yard for an entire season to get rid of it without chemicals.