According to the California New Car Dealers Association (CNCDA), some 6,480,506 vehicles were sold in the country in H1 2020, which is 23.5% less than a year ago (8,471,317).
Tesla's market share in California is at 4.2% (33,083 cars), while nationwide it is at 1.4%. Comparing it with the total volume, we can estimate that the total Tesla volume in the U.S. was roughly 90,750 (± up to 3,250: probably within 87,500-94,000). We bet that it's closer to 87,500 if not less.
It would mean that sales outside of California stand at 57,667 (probably within the 54,000-60,900 range). California's share would be then 36.5%.
Compared to H1 2019
A year ago, CNCDA reported Tesla's market share of 1.1%, so the volume was estimated at 92,250 (93,200 using the corrected data from the new report). Again, the real number might be higher or lower by up to 4,250 (within 88,000-96,500).
Assuming that the data is valid, Tesla sales fell in the U.S. by just a few percent (3% nominally or by up to roughly 10% in the worst-case scenario).
Regardless of the precise sales numbers, Tesla remains the major force behind car electrification in the U.S.