Liberal activists are urging U.S. Supreme Court Justice Anthony Kennedy, a conservative, with whom they often disagree, to put off any thought of retirement, fearing President Donald Trump would replace him with a jurist further to the right.

The liberal Democrats’ keep-Kennedy campaign, being pursued publicly and privately, reflects how powerless they have become against the Republican president when it comes to high court vacancies since the Senate in April reduced the vote tally needed to confirm a Supreme Court nomination to 51 from 60.

It also shows how big the stakes are for both sides in any decision that Kennedy, who turns 81 in July, makes about his future on the court. If he were to retire, Trump would have a historic opportunity to recast the court in a more conservative posture, possibly for decades to come.

Some former Kennedy clerks have said he is thinking about retirement. He has declined to comment on his plans, despite requests from many media outlets including Reuters.

Right now, Kennedy “is the most important man in America. He is the vote that swings the court on the most important cases that reach it,” said Elizabeth Wydra, president of the Constitutional Accountability Center, a left-leaning think tank.

Nominated by Republican President Ronald Reagan in 1987 to a lifetime court seat, Kennedy has been a crucial swing vote on the nine-member court for more than a decade.

On most issues, such as campaign finance and religious rights, he has voted with fellow conservatives. He also voted with the minority to strike down the 2010 Affordable Care Act, popularly known as Obamacare. But on gay rights and abortion, he has sided with the court’s four liberals.

If he stays in his post, the court’s long-standing ideological balance will be preserved. If he quits, Trump could replace him with someone who tilts further right, giving conservatives a solid five-vote majority.

Wydra and other liberals are lionizing Kennedy and his legacy in the media. Some are reaching out to former Kennedy clerks and others who know him, asking them to urge him not to retire, said Michele Jawando, a legal advocate at the Center for American Progress think tank in Washington.

One former Kennedy clerk confirmed being asked to urge him to stay on and said other clerks had asked him to do so. Other clerks said they had not been approached by liberal activists.