Excellent article on link below. Be sure to heed the warning regarding Hot Weather.
How to kill Crabgrass

Note what that article says about many Crabgrass poisons also killing other grass species, you may make the situation worse by killing off the Crabgrass, and that is a temporary fix anyway since if the conditions that allow that Crabgrass to grow are not corrected it will be back next year.. With as much Crabgrass as you say you have it probably is not a good idea to try to kill those plants now anyway. But you can get started on working to lessen the invasion next year by working to making your lawn into a strong and healthy one that could resist Crabgrass better.

Start with a good soil test. If your state universities Cooperative Extension Service offers one that is the best place to go and you get the help and advice of the county Horticultural Agent.

Along with that soil test for soil pH and major nutrients these simple soil tests may be of some help as well.

1) Soil test for organic
material. From that soil sample put enough of the rest to make a 4 inch level in
a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and
replace the lid, tightly. Shake the jar vigorously and then let it stand for 24
hours. Your soil will settle out according to soil particle size and weight.
For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the
bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above
that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole
1 foot square and 1 foot deep and fill that with water. After that water drains
away refill the hole with more water and time how long it takes that to drain
away. Anything less than 2 hours and your soil drains’ too quickly and needs
more organic matter to slow that drainage down. Anything over 6 hours and the
soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful
of your slightly damp soil and squeeze it tightly. When the pressure is
released the soil should hold together in that clump, but when poked with a
finger that clump should fall apart.

4) Smell. What does your
soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant
odor? The more organic matter in your soil the more active the soil bacteria
will be and the nicer your soil will smell, to a point. Too much organic matter
can be bad as well.

5) Life. How many
earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5,
according to the Natural Resources Conservation Service, indicates a soil that
is not healthy.

Once you have a good idea of what the soil needs to grow a strong and healthy lawn you can start working on that for seeding in the early fall with a grass species more desirable.