New York, Massachusetts, and Vermont may postpone for four years a requirement that automakers increase sales of electric cars to improve air quality. Two years ago, the states adopted California’s standard, which mandates that by next year, 8 percent of cars sold must be much cleaner than current cars and another 2 percent must be entirely emissions-free (i.e., electric cars). The proposed delay in the Northeast would cut the market for electric cars roughly in half; environmentalists say the shift would critically harm efforts to make such cars affordable. Top environmental officials in the three states, however, say the technology doesn’t yet exist to make electric cars viable, and that focusing their efforts on other strategies to reduce tailpipe emissions would actually result in cleaner air.