The justices of the U.S. Supreme Court gather for a group portrait at the Supreme Court Building in Washington in 2010. (Larry Downing/Reuters)

The Supreme Court starts the new year Monday with a politically charged battle over organized labor, only one of the controversies that are putting the ideologically divided and aging justices at the center of the presidential campaign.

Already on the docket are abortion, affirmative action, the rights of religious objectors to opt out of legal obligations, and a clutch of election-law disputes that could benefit one political party over another. The court will probably soon add a review of President Obama’s executive actions aiming to shield millions of undocumented immigrants from deportation.

The agenda provides a dramatic confluence of a Supreme Court term with a presidential election.

“It’s a fascinating moment,” Neera Tanden, president of the liberal Center for American Progress, said in a panel discussion last week. “Because we are entering 2016, and I think the court has the potential to place itself in the heart of the political debate that we are going to have in the country.”

[Supreme Court case could curtail union power]

Rebecca Friedrichs is part of a group of California teachers challenging the state’s union. “Every individual should have the right to decide” which organization she supports, Friedrichs says. (Greg Schneider/AP)

The court’s recent decisions upholding the Affordable Care Act and ruling that gay couples have a constitutional right to marry already have set the terms for debate in the presidential campaign. Its ruling in Citizens United v. FEC six years ago opened the door for the record-making amounts of money spent on the race and how it can be raised.

The coming decisions will probably be as closely divided as those were.

But there is a striking difference this term, compared with the last one. In its most important case, the court’s liberals joined with Justice Anthony M. Kennedy to issue a landmark ruling that the Constitution does not allow states to recognize marriages only between one man and one woman.

This year, the liberals will be playing defense, trying to defend precedents and preserve the status quo.

The court’s current split will be on display Monday. The justices will hear a challenge from a group of California teachers who say it violates their First Amendment rights to be forced to pay dues to the state’s teachers union.

[Two teachers explain why they want to take down their union]

The state is one of about 20 in which public employees are required to either join the union or pay a fee to support the union’s collective-bargaining activities. The unions say this is only fair because they are required to negotiate on behalf of all workers, not just their own members.

A supporter of gay marriage waves a rainbow flag in front of the Supreme Court Building in Washington in April. (Astrid Riecken/For The Washington Post)

The non-members are not required to subsidize the union’s political activities, and the Supreme Court approved such a system in 1977, turning down a challenge that it was a violation of the fee-payer’s speech and association rights.

But in a 2014 case involving health-care workers from Illinois, four of the court’s conservatives signed onto an opinion from Justice Samuel A. Alito Jr. that called the decades-old decision, Abood v. Detroit Board of Education, “questionable on several grounds.”

[Supreme Court deals blow to unions and signals bigger decision ahead]

Rebecca Friedrichs and other California teachers have given the majority a clear chance to overrule the precedent. “Every individual should have the right to decide” which organization she supports, said Friedrichs, a third-grade teacher from Anaheim whose case is championed by conservative legal organizations.

Being excused from paying for the union’s political activities is not enough, she said. Public-employee union negotiations necessarily affect public policy decisions on government spending and taxes and issues such as seniority and educational policy.

Labor leaders call the challenge a “radical” attack on public-employee unions, one of the strongest segments of the organized labor movement. Laura Juran, a lawyer for the California Teachers Association, said teachers who disagree with the union’s views “are perfectly free to speak as citizens” to try to influence government decisions.

The partisan significance of the case is clear. Public-employee unions have become a major player in Democratic politics, generating campaign contributions and on-the-ground support for candidates. At the same time, disputes between the unions and Republican governors have become frequent and bitter.

The justices acknowledged the partisan stakes during oral arguments in the 2014 case. Justice Elena Kagan mentioned the tensions that flared in Wisconsin after Gov. Scott Walker (R) moved to curtail union rights and union members led an unsuccessful recall attempt. Nevertheless, she defended the current system in which it is up to states to decide whether “agency shop fees” are mandatory.

Alito, in the same argument, mentioned how some politicians are indebted to the unions. The Illinois case was prompted by a decision by disgraced former governor Rod Blagojevich (D) to reward the Service Employees International Union by declaring its workers to be state employees for collective-bargaining purposes.

“I thought the situation was that Governor Blagojevich got a huge campaign contribution from the union, and virtually as soon as he got into office, he took out his pen and signed an executive order that had the effect of putting, what was it, $3.6 million into the union coffers?” Alito asked Solicitor General Donald B. Verrilli Jr.

In both that and the current case, Verrilli and the Obama administration intervened on behalf of the unions and argued against overturning the precedent.

The union case and the other controversies underline the importance of the coming presidential election in determining the court’s future.

A president’s ability to build a legacy by shaping the makeup of the Supreme Court is an important consideration in every election, but especially this year. On Inauguration Day 2017, Justices Antonin Scalia and Kennedy will have joined Ruth Bader Ginsburg in their 80s, and Stephen G. Breyer will be 78.

If Ginsburg’s seat is filled by someone selected by a Republican president, this could consolidate a conservative majority independent of Kennedy, who has at times been a key swing vote. A Democrat replacing either Scalia or Kennedy would create a new five-justice majority that consistently leans to the left.

“That makes this a make-or-break moment — for the court and our country,” Democrat Hillary Clinton wrote in an op-ed Friday in the Boston Globe.

“As president (and a lawyer and former law professor), I’ll appoint justices who will protect the constitutional principles of liberty and equality for all, regardless of race, gender, sexual orientation or political viewpoint; make sure the scales of justice aren’t tipped away from individuals toward corporations and special interests; and protect citizens’ right to vote, rather than billionaires’ right to buy elections.”

Republican rival Sen. Ted Cruz (Tex.), a former Supreme Court clerk and practitioner, envisions a different court if it is a President Clinton who is making the nomination to replace, for instance, Scalia.

[Cruz is a former Supreme Court clerk but no longer a friend of the court]

In campaign ads and speeches in Iowa, Cruz has warned of a Supreme Court intolerant of religious freedom and expression.

“We will see Ten Commandments monuments all over this country torn down, because a radical Supreme Court says we cannot acknowledge the Ten Commandments,” Cruz said in a speech last week in Rock Rapids. “If Hillary Clinton is elected president, we will see a radical Supreme Court ordering veterans’ memorials to be taken down all over this country. We are just steps away from the chisels coming out in Arlington, to remove crosses and stars of David from the tombstones of our fallen soldiers.”

[Supreme Court takes up major challenge to abortion restrictions]

Many of the court’s decisions on issues that could shape the 2016 political landscape will come just weeks before the Republican and Democratic conventions in July.

The justices hear arguments in March on a Texas abortion law. If it wins the court’s approval, the decision would probably lead to restrictions on abortion that have previously been struck down by lower courts citing earlier Supreme Court precedents.

In another case argued late last year, the limited use of race by the University of Texas in the admission process has been challenged. University leaders say this use is consistent with the court’s 2003 ruling on affirmative action. A decision is still pending.

And the rights of religious objectors would be enhanced if the court agrees in the coming months that they should be exempted from the requirement to provide their employees with contraceptive services under the Affordable Care Act.

With the future membership of the Supreme Court in doubt, said Walter Dellinger, a liberal Supreme Court practitioner and former Clinton administration lawyer, conservative legal activists are swinging for a “home run” now.

David Weigel contributed to this report.