This week “The Economist explains” blog looks at the American presidential candidates' positions on major policy issues. For four days until Thursday this blog will publish a short explainer about one specific area

THE Supreme Court is a campaign issue every time America elects a president. The power to fill vacancies on the court (which the White House shares with the Senate) is nothing to sniff at, and justices are among presidents’ most lasting legacies. But in most election cycles, the immediate future of the Supreme Court is hazy: nobody knows when a justice will die or decide to retire. This time, with Republican senators’ obstruction keeping Antonin Scalia’s seat empty since his death in February, the matter is anything but abstract. Donald Trump warns that America is destined to become another Venezuela if Hillary Clinton is empowered to choose justices. Mrs Clinton, meanwhile, says a Supreme Court shaped by her opponent would doom “the future of the planet”. Hyperbole aside, how will the election affect the trajectory of the Supreme Court?

The demographics of the Court suggest that the next president could appoint as many as four justices, including a replacement for the very conservative Mr Scalia. Ruth Bader Ginsburg, the leader of the court’s left wing, is 83. The slightly less liberal Stephen Breyer is 78, and Anthony Kennedy, who sits in the ideological middle of the bench, is 80. Each of the four most recent presidents has seated two justices, so having the chance to name four would give the next White House an outsize influence on the shape of American law for a generation. Yet even if these three jurists, along with their five younger colleagues, spend four more years in their robes and the number of vacancies stays at one, the next president will still help determine the shape of the Supreme Court for years. With the remaining eight members of the court split neatly along ideological lines, the 113th justice is certain to swing the court one way or another.

Mr Trump has identified 21 people he says he would consider to fill Mr Scalia’s seat; any of them would prolong a conservative tilt that has defined the Supreme Court for nearly half a century. But if Mrs Clinton prevails in November, it is likely the court will soon have a majority of justices appointed by Democratic presidents, for the first time since 1970. There are several ways this might happen. Republicans may relent after November 8th and quickly confirm Barack Obama’s centre-left nominee, Merrick Garland of the DC Circuit Court of Appeals, in the hopes of denying Mrs Clinton the chance to pick her own justice. If Mr Garland remains in no-man’s land, Mrs Clinton may re-nominate him in January. Or, if the Democrats retake the Senate, she may be emboldened to choose someone younger (Mr Garland turns 64 in November) and more liberal.

In recent weeks, Senate Republicans have signalled another possibility: indefinite stalemate. Before softening the message a few hours later, John McCain said on October 17th that Republicans “will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up”. Ted Cruz noted, accurately, that there is "long historical precedent for a Supreme Court with fewer justices" (though the number has been set at nine since 1869). If the Senate deigns to consider and confirm a nominee, do not expect changes overnight. Mrs Clinton says she will choose a justice who is committed to overturning the controversial 2010 Citizens United decision that significantly loosened campaign-finance rules. Some on the left hope a liberal majority would also set to work undoing recent rulings that have gutted the Voting Rights Act and expanded gun rights. But the Supreme Court is not a roving, agenda-driven commission. It is a slow-moving institution empowered only to decide cases that work their way up through the lower courts, which makes quick about-faces on these conservative rulings unlikely. In the medium and longer terms, however, a majority of Democratic appointees may reshape American constitutional law in dramatic terms—rolling back decades of conservative jurisprudence on matters including racial equality, church and state, private property and the death penalty.

Also in this series

Fiscal policy

Climate change

Foreign policy

Over the past several weeks The Economist has run two-page briefs on major areas of American policy. Read the full brief on the Supreme Court here. A free pdf containing all seven briefs is available here