WASHINGTON — Since the death of Justice Antonin Scalia last February, the Supreme Court has been evenly divided between Democratic appointees and Republican ones. This has resulted in occasional deadlocks and many narrow decisions.
A new justice appointed by President Donald Trump would revitalize the court's conservative bloc. The balance of power would then return to the one in place since 2006: leaning right, but tempered by the occasional liberal votes of Justice Anthony M. Kennedy.
Trump said in a Twitter post Monday that he would announce his pick today at 8 p.m.
Here is a look at how a court with a reinvigorated conservative wing might act.
For now, abortion rights appear secure
So long as Kennedy continues to support the broad outlines of the constitutional right to abortion established in 1973 in Roe vs. Wade, the addition of a new justice is not likely to make a difference.
In June, Kennedy reassured supporters of abortion rights on this point, joining the court's four-member liberal wing to strike down parts of a restrictive Texas abortion law by a 5-3 vote. A new justice opposed to abortion rights would tighten that tally but not tip it.
Kennedy had once before voted to find an abortion restriction unconstitutional, in Planned Parenthood vs. Casey in 1992, when he joined Justices Sandra Day O'Connor and David H. Souter to save the core of Roe vs. Wade.
All three of those justices were Republican appointees who, on this issue at least, disappointed some of the conservatives who had supported their nominations. Trump has said that his nominee will vote to overturn Roe.
Affirmative action seems safe, too
In June, the court upheld a race-conscious admissions program at the University of Texas by a 4-3 vote. Justice Elena Kagan was recused but would almost certainly have voted with the majority, making the effective vote 5-3. Here again, a single Trump appointment would not tip the balance.
Kennedy, the author of the majority opinion, has long been skeptical of race-sensitive programs and had never before voted to uphold an affirmative action plan. He dissented in the last major affirmative action case.
There are other cases in the pipeline, including one challenging Harvard's admissions policies. But Kennedy is unlikely to retreat from his position in the Texas case, and a new four-member conservative bloc would probably not vote to add an affirmative action case to the court's docket unless its members thought they could attract Kennedy's vote.
Public sector unions should be worried
A threat to public sector unions that ended in a deadlock in March could soon reach the court again. This time, the challengers are likely to gain a fifth vote.
When the case was argued in January 2016, before Scalia died, the court's conservative majority seemed ready to say that forcing public workers to pay fees to unions they had declined to join violated the First Amendment. Scalia's questions were consistently hostile to the unions.
A ruling allowing workers to avoid paying fees for activities like collective bargaining has been the subject of a decades-long campaign by conservative groups that seek to weaken unions representing teachers and other public employees. A new Trump appointee would probably hasten such a ruling.
Environmental regulations may be in peril
A few days before Scalia died, the Supreme Court's five-member conservative majority blocked President Barack Obama's ambitious effort to combat global warming by regulating emissions from coal-fired power plants.
The Supreme Court had never before granted a request to halt a regulation before review by a federal appeals court. The appeals court in this case, the U.S. Court of Appeals for the District of Columbia Circuit, heard arguments in September, and it is expected to rule soon.
But the stay issued by the Supreme Court will remain in place until, at the earliest, it decides whether to hear an appeal from the appeals court's ruling should the losing side seek review. Once the court's conservative wing is back at full strength, environmental groups will have reason to be nervous.
Contraception , transgender cases may fizzle
Other cases concern actions by the Obama administration that may be reversed under Trump. If that happens, the cases they gave rise to may not reach the court or may be dismissed.
In May, for instance, the court unanimously returned a series of cases on access to contraception to the lower courts for further consideration in the hope that the two sides could settle their differences. That seemed unlikely given the divide between religious employers on one side and the Obama administration on the other, and at least some of the cases were widely expected to return to the Supreme Court in one form or another.
The Trump administration may well accommodate the religious groups' objections, removing the need for further Supreme Court review.
Something similar may happen in a case on transgender rights on the Supreme Court's docket. The case revolves around the Obama administration's interpretation of a federal regulation under a 1972 law that bans discrimination "on the basis of sex" in schools that receive federal money. Here, too, the Trump administration may offer a different interpretation of the regulation, raising the possibility that the justices would dismiss the case.
This may not be Trump's only appointment
Another vacancy during Trump's presidency is entirely possible. The three oldest members of the court are Kennedy, 80, and the senior members of its liberal wing, Justice Ruth Bader Ginsburg, 83, and Justice Stephen G. Breyer, 78.
Should Trump have the opportunity to replace one of those three justices, the court would in all likelihood be transformed into one with a solid five-justice conservative majority. It would enter a new phase, and liberal precedents on abortion, the death penalty and gay rights would be at risk.
Indeed, a second appointment to the Supreme Court could be Trump's most lasting legacy.