President Clinton has threatened a veto for the third time. But, nevertheless, the Senate this month passed a bill that would expand tax credits for employer-provided educational assistance and increase from $500 to $2,000 a year the amount of money an individual could set aside tax-free for future education expenses. Much of the controversy centers on the portion of the bill that reduces the restrictions on how that money could be used. Currently, money in education savings accounts can be used only for college expenses. The current proposal would allow people to use such accounts for both public and private elementary or high school education expenses also.