President Reagan's Supreme Court nominations promise important change in this country's reading of its law, and in the balancing of its people's rights and duties. The number of justices conventionally counted as conservatives will not increase, but the nature of their conservatism may develop quite differently. Under a Chief Justice Rehnquist, with the addition of Antonin Scalia to the court, the new majority seems likely to be more aggressive in its views. These appointments give a sense that a new definition of constitutional conservatism may be emerging, one with a sharper edge than its predecessor.

Chief Justice Burger, in his long tenure on the court, has been counted as a conservative, but he was very conscious of a series of legal and social issues -- of which racial desegregation was the most important -- that are now fading into the past. He wrote the unanimous opinion in the Nixon tapes case. He was the author of landmark opinions in school busing and employment discrimination cases and voted with six colleagues to legalize abortion. A modest presence in the law, he gave over much time and effort to administrative problems that needed correcting.

Both Justice Rehnquist and Judge Scalia, of the U.S. Court of Appeals for the D.C. Circuit, are well known as sitting judges and as serious conservatives. Opponents of some of President Reagan's judicial appointments often cite them as the kind of appointments they would probably vote to confirm. Both share the president's political point of view.

We think they are wrong on many things. Justice Rehnquist, during his tenure on the court, has favored tuition tax credits for parochial schools and residency requirements for welfare beneficiaries. In the Bob Jones case, he alone dissented, writing that the IRS could not, without statutory authority, deny tax exemptions on the basis of discrimination. He has been a firm opponent of Roe v. Wade and its successor cases on abortion and a strong defender of the death penalty. Judge Scalia's views of the First Amendment, as expressed in his opinions, are extremely troubling. (We should note that he joined in an opinion adverse to this newspaper in a pending libel case.) Both men have, in our opinion, demonstrated an alarming insensitivity to civil liberties and the Bill of Rights.

The rough balance of votes on most issues is not likely to shift. But the content of contemporary conservatism is not a quiet or settled thing. New issues arise and, along with them, new ways of looking at the law. The currents associated with the rise to political power of Ronald Reagan are not entirely new to the law, but they are now going to be given powerful reinforcement at the peak of the country's legal and constitutional structure.