As Washington prepares to go into lockdown for the largest gathering of world leaders hosted by a US President since the founding of the United Nations in 1945, President Barack Obama might be thinking of things other than nuclear security.

Following the retirement of Justice John Paul Stevens on Friday, Obama will spend the coming weeks picking his second nominee for the US Supreme Court. Apart from the opportunity to cement his legacy, the choice could spark an ugly confirmation battle in the run-up to November’s congressional elections.

Already, a front-runner is emerging. Obama is said to be revisiting his shortlist from year ago, when he successfully nominated Sonia Sotomayor to the court. Runner-up last time was Solicitor General Elena Kagan, whose career in government and academia means she has no judicial experience to give clues to her legal philosophy.

Kagan, 49, has always been something of a ground-breaker. Her resume includes study at Princeton, Oxford and Harvard, and a clerkship with Supreme Court Justice Thurgood Marshall. She is the first female to serve as the solicitor-general, sometimes referred to as “the 10th justice” and responsible for representing the government before the Supreme Court. She worked in the Clinton administration as associate counsel to the president and as a domestic policy adviser. In 2003 she was appointed dean of Harvard Law School, the first woman to hold the position.

Her 2009 confirmation process as solicitor-general provides some clues on what to expect from Justice Kagan. She endorsed the legal concept of stare decisis, arguing courts should pay deference to decisions that have become settled law, a doctrine embraced by opponents of judicial activism.

Amy Howe, editor of SCOTUSblog, which reports on the court, told Crikey that Kagan “should be considered a strong possibility” for the nomination, but that she would not alter the philosophical balance of the court.

“Ideologically there won’t be much of a difference to the court in that Obama will be replacing a liberal with a liberal, but as leader of the court’s liberal wing Justice Stevens played an difficult role,” she said. “It will be hard for whoever is nominated to play the same role.”

While at Harvard Kagan was seen as an innovative administrator, recruiting high-profile faculty from both sides of the ideological divide including several Bush administration lawyers. She attracted controversy in 2004 when she refused military recruiters access to campus facilities to protest the Don’t Ask, Don’t Tell policy, banning gay and l-sbian soldiers from serving openly in the US military. She reversed the ban two years later when a Supreme Court decision left the school at risk of losing federal funding.

Howe said Kagan’s initial opposition to DADT could be explored at her Senate confirmation hearing: “I think it will be relevant should Kagan be nominated, because people who are opposed to her could seize on that issue and make her life difficult.”

Kagan made her first oral argument before the court last September in the Citizens United case. She unsuccessfully asked the court to uphold campaign finance laws but, in a 5-4 ruling, the court struck down laws that ban political expenditures by corporations in candidate elections.

Obama is also said to be considering two US appeals court judges, Diane Wood and Merrick Garland, as well as several political choices, as bold as Secretary of State Hillary Clinton.

Despite the Democrats having 59 Senate seats, Republicans have already refused to rule out a filibuster to try to block a nominee they see as a liberal activist. Senator Jeff Sessions, ranking Republican on the powerful Judiciary Committee, said on Meet the Press Sunday: “If it’s somebody like that, clearly outside of the mainstream, then I think every power should be utilised to protect the constitution.”

If the nomination gets bogged down in Washington’s usual brand of bipartisanship it could be a long, hot summer for Obama and the next Justice of the US Supreme Court.