* Conservative Gorsuch expected to cast deciding vote
* High court heard similar case in 2016, but split 4-4 (Recasts first paragraph, adds details from arguments)
WASHINGTON, Feb 26 (Reuters) - In a case that could weaken the finances and political clout of organized labor, conservative U.S. Supreme Court justices on Monday indicated strong support for stopping millions of dollars in fees that non-members are forced to pay annually to unions representing public employees.
But the justice whose vote is likely to decide the case, President Donald Trump's appointee Justice Neil Gorsuch, remained silent throughout the one-hour argument. His reputation as a staunch conservative suggests he will join his fellow conservatives in an eventual 5-4 ruling against the unions.
The court's liberals expressed sympathy toward retaining the so-called agency fees. Workers who decide not to join unions representing police, teachers, firefighters and certain other state and local employees must pay the fees in two dozen states in lieu of union dues to help cover the cost of non-political activities such as collective bargaining.
Roughly 5 million public-sector workers pay such fees. A Supreme Court ruling disallowing these fees would deal a setback to an already-diminished American organized labor movement, taking away a vital revenue stream from unions and undercutting their ability to attract new members.
Dumping the fees would require the Supreme Court to overturn a 41-year-old precedent that allowed the payments.
Lawyers for Mark Janus, the Illinois state child-support specialist who is the plaintiff in the case, argued that forcing non-members to pay these fees to unions whose views they may not share violates their rights to free speech and free association under the U.S. Constitution's First Amendment. A lower court ruled against Janus, setting up the Supreme Court showdown.
Conservative justices Anthony Kennedy and Samuel Alito were particularly forceful in questioning the lawfulness of the fees, based on the notion that collective bargaining is in essence a political activity. Unions argue that their political activities are separate from negotiating contracts.
"If you do not prevail in this case, the unions will have less political influence. Yes or no?" Kennedy asked union lawyer David Frederick.
"Yes, they will have less political influence," Frederick said.
"Isn't that the end of this case?" Kennedy said.
Kennedy suggested that negotiations between states and unions at the bargaining table have a direct impact on public policy, which can include bigger government budgets, increased public debt and higher taxes.
"Doesn't it blink reality to deny that that is what's happening here?" Kennedy asked.
Alito focused on the right of non-members not to be forced to subsidize speech they disagree with, calling this a more serious problem than restricting speech.
"When you compel somebody to speak, don't you infringe that person's dignity and conscience in a way that you do not when you restrict what the person says?" Alito said.
Liberal Justice Elana Kagan stressed how disruptive a ruling against the unions would be, leading to thousands of labor contracts being renegotiated, striking down laws in various states and affecting "the livelihoods of millions of individuals."
"When have we ever done something like that? What would be the justification for doing something like that?" Kagan asked.
SIMILAR CASE IN 2016
The justices heard a similar case in 2016, and had appeared ready to throw out the fees and overturn the high court's 1977 precedent that allowed them. But the death of conservative Justice Antonin Scalia then left the court with an even split of conservatives and liberals, and its 4-4 decision in March 2016 failed to settle the legal question.
Republican President Donald Trump's appointment of Gorsuch last year restored the Supreme Court's 5-4 conservative majority.
A ruling is due by the end of June.
Unions contend that mandatory agency fees are needed to eliminate the problem of what they call "free riders" -- non-members who benefit from union representation, for example through salary and working conditions obtained in collective bargaining -- without paying for it.
Janus, the plaintiff, opted not to join the union that represents employees like him, the American Federation of State, County and Municipal Employees (AFSCME), and sued the union over the fees.
Federal employee unions cannot collect agency fees. A ruling against the unions would not directly affect private sector unions.
Liberal Justice Sonia Sotomayor appeared to favor the argument made by unions and states defending the fees that the government has a compelling interest in allowing unions to represent workers, in part because it provides a mechanism for labor disputes to be resolved.
"Why isn't that a compelling interest?" she asked.
Depriving unions of agency fees could undermine their ability to spend in political races. They typically back Democratic candidates over Republicans.
Dueling groups of protesters gathered outside the white marble courthouse ahead of the argument.
Union-backed protesters held signs saying "America needs union jobs," while those supporting the challengers had signs saying "stand with Mark," a reference to the plaintiff in the case, Illinois state worker Mark Janus. (Reporting by Lawrence Hurley; Additional reporting by Robert Iafolla and Andrew Chung; Editing by Will Dunham)