Ah, the joys of Memorial Day weekend: the de-facto start of summer, when college is over, when backyard barbecues are readied for action, when public swimming pools are opened and – this year – when a nation waits with bated breath to learn who President Obama will choose to fill his first vacancy on the Supreme Court.

Well, perhaps that last is an exaggeration. The make-up and ideological tilt of the highest legal authority in the US probably isn't the hottest topic of conversation right now at a cook-out in Kentucky, or on the mean streets of south central LA. But in Washington DC it is.

Indeed, ever since David Souter announced on 1 May that he would be retiring at the end of the court's current term next month at the comparatively young age of 69, speculation has been of little else. Legal experts have their shortlists of candidates. Activists and special interest groups are on a war footing. The media scrutinise more closely than ever the comings and goings at the White House, to divine who might have been discreetly summoned for a job interview. The blandest presidential utterances are parsed for clues.

And now the anticipated moment is at hand. Obama will spend the holiday weekend at Camp David pondering his options. And later this week, he may make known his decision.

For non-Americans, the fuss is hard to understand. After all, this isn't a presidential election, just the selection of one judge among nine. Is the entire circus no more than proof, yet again, that the US is the most legalistic, court-obsessed country on earth?

Would that it were so simple. In fact, a president's right to appoint a justice is, apart from his role as commander-in-chief, arguably his greatest power, and certainly his most enduring. The Supreme Court isn't just the highest court of appeal in the land; it's one of the three "co-equal" branches of the US government, and in terms of its impact on Americans' daily lives, probably ranks second, behind the presidency, but ahead of a partisan and often deadlocked Congress.

As guardian of the constitution, it can uphold or remove individual rights, and strike down laws. It was the Supreme Court, not Congress or the White House, that gave wings to the civil rights movement by outlawing segregated schools in 1954.

A president, moreover, serves eight years at most. But judgeships are for life. To the relief of most of his compatriots, George W Bush is now confined to working on his memoirs. But his influence will linger indirectly for decades, in the persons of John Roberts and Samuel Alito, the two conservative justices he selected, and who, on recent trends, could easily still be serving in 2030.

In the 220-year existence of the Supreme Court, there have been only 110 justices. Fourteen served for three decades or more – most recently John Paul Stevens, appointed by President Ford in 1975 and still going strong at the age of 89. If he can make it to July 2012, he will be the longest-serving justice in US history.

For all his longevity, though, Stevens couldn't prevent Bush from enjoying what one legal scholar here calls the coveted "trifecta", when the same party controls all three branches. Even after Obama's nominee is in place, six of the nine justices will have been appointed by Republican presidents.

And while Democrats run the White House and Congress, the third leg of the trifecta will probably elude Obama, barring an act of God. The two justices considered most likely to step down next – Stevens and the court's lone female member, Ruth Bader Ginsburg – are both liberals. Not only are the conservatives mostly younger; they will be strongly inclined to put off any retirement until after 2012, or 2016 if Obama wins a second term, in the hope of having their successor picked by a Republican president.

So who will it be? The smart money says a woman, almost for sure, and possibly a Latino to boot. Among the names circulating are Diane Wood, a Chicago judge and one-time teaching colleague of Obama at the University of Chicago law school; Elena Kagan, the solicitor general; and Sonia Sotomayor, a federal appeals court judge rumoured to be seeing Obama at Camp David this weekend. Others mentioned include Michigan's governor, Jennifer Granholm, and Janet Napolitano, the homeland security secretary.

And, for the record, there's another one: a brilliant former graduate of Yale Law School named Hillary Clinton. No one seriously expects her to be asked. But the very notion that she might be tempted is a measure of the job's importance. After all, secretaries of state come and go, but Supreme Court justices go on (almost) forever.