Crabgrass is not a perennial, but rather an annual grass, meaning that it does not live over the winter and that each spring all the Crabgrass that you have in your lawn has grown from seed from last year's crop. By contrast, Twitchgrass is virtually self-sterile and grows only from very prolific rizome roots. These differences are important when attempting to eradicate weed grasses, because it narrows down the best methodology.
Because Crabgrass produces literally thousands of seeds from each mature plant, pre-emergence chemcial treatments are at best, a control, a means of reducing the onslaught of the current year's growth and, in fact, create more of a problem in leaving one with the hope that it will actually eliminate Crabgrass. After attempting such a control, year-over-year, it becomes abundantly clear that the Crabgrass is so prolific that despite the pre-emergence applications, the Crabgrass is quickly and completely replacing all the desirable grass in the lawn.
Other than pre-emergence controls, there is no other method of killing Crabgrass that will not also kill all other grasses in your lawn. Therefore, the reality is that the only way to eliminate Crabgrass is by first completely killing the whole lawn area in which it resides, with a herbiside such as Roundup and then, as quickly as possible, so as not to allow other weed seeds to take over the open soil area, reseed the whole lawn using at least twice as much seed as is recommended for growing a new lawn (seed needs to be racked into good soil, along with starter fertilizer and kept well-watered morning and evening until germinated).
By far the best time to do this is early fall (depending on growing zone - last August to early September) when weed seed germination is at its lowest and the summer drying heat is past, but with sufficient time before winter to allow for good germination.
The next thing to understand is that Crabgrass will be at its height in seed production in early August, right at the time when you need to be killing it with Roundup, but take heart, my experience is that despite the hundreds of thousands of seeds in production at this time, if you drench it with Roundup, it also kills most of the seeds, which is the primary reason why simply killing all the grass - along with all its seed production - is really the ONLY way of really getting control of a bad patch of Crabgrass. So get on with it and then simply reseed and check the next spring (late May/early June - before the seed forms) for young Crabgrass sprouts that will inevitably have dodged the Roundup and meticulously hand pull (very easy when young) each sprout you see. Repeat this spring process ever year and throughout the summers until you see no more Crabgrass. And yes, that process, tedious as it is, absolutely does work! Best wishes for your success!!
S. Moore