The federal tax credit was built to spur sales of electric vehicles. When Tesla reached the 200,000 vehicles delivered mark, the phaseout began, and the first phase will end for them on December 31. This means that any Tesla vehicles delivered before then will still qualify for the full $7,500 tax credit. Tesla has stated that any car ordered by November 30 will be delivered before the end of the year.
If you are looking to buy a Tesla, now is the time to maximize value. If your car is delivered between January 1 and June 30, you will qualify for a $3,750 credit when you file your taxes the following year. If you get your car between July 1 and December 31 of 2019, you will qualify for $1,875, and after that, nothing.
Also remember that last year congress attempted to do away with the tax credit completely, so there is a chance they could try to kill it off again, which would mean there is no more credit after December 31. This seems unlikely, as GM is lobbying hard to actually extend the tax credit, since they will be reaching 200,000 as well this year, meaning their phaseout is just one quarter behind Tesla.
One other thing to consider is if you do not have $7500 in tax liability for the year, then you will not be able to take advantage of that anyway.
Tesla cannot guaratee delivery by December 31 on every car and configuration, but there are some that are available. You can talk to a sales representative and find a car that works for you that is also available to delivery quickly. A mid range Model 3 will most likely deliver before the end of the year still, depending on color options. Likewise, a Performance Model 3 can also be purchased before then. If you are looking for non Performance dual motor car, then that is less likely especially if you are on the east coast. Model S and X are also going to be tougher to get that soon, but some are available, and if you are interested in buying a slightly used showroom car, those also qualify for the tax credit.
If you are seriously looking, let me know via Twitter @McHoffa and I'll put you in touch with a very helpful sales employee! If you need a referral code for six months of free supercharging, that is available below.