WASHINGTON, March 18 (Reuters) - The U.S. Supreme Court

on Monday marches back into the battle over social media content

moderation in a challenge on free speech grounds to how

President Joe Biden's administration encouraged platforms to

remove posts that federal officials deemed misinformation,

including about elections and COVID-19.

The justices are set to hear arguments in the

administration's appeal of a lower court's preliminary

injunction constraining how White House and certain other

federal officials communicate with social media platforms.

The Republican-led states of Missouri and Louisiana, along

with five individual social media users, sued the

administration. They argued that the government's actions

violated the U.S. Constitution's First Amendment free speech

rights of users whose posts were removed from platforms such as

Facebook, YouTube, and Twitter, now called X.

The case tests whether the administration crossed the line

from mere communication and persuasion to strong arming or

coercing platforms - sometimes called "jawboning" - to

unlawfully censor disfavored speech, as lower courts found.

Biden's administration has argued that officials sought to

mitigate the hazards of online misinformation, including false

information about vaccines during the pandemic that they said

was causing preventable deaths, by alerting social media

companies to content that violated the platforms' own policies.

The justices in February heard arguments in another social

media case over whether to uphold laws passed in Texas and

Florida that would restrict the content moderation practices of

platforms.

The Republican backers of those laws have voiced concern

over what they portray as bias against conservative voices on

these platforms. Many researchers, as well as liberals and

Democrats, have warned of the dangers of social media platforms

amplifying misinformation and disinformation about public

health, vaccines and election fraud.

In the case being argued on Monday, the plaintiffs have

argued that platforms suppressed conservative-leaning speech,

which they attribute to government coercion, a form of state

action barred by the First Amendment.

The Supreme Court in October put the lower court's

injunction on hold pending the review of the case by the

justices.

The Justice Department said that government officials,

including presidents, long have used the bully pulpit to express

views and to inform on matters of public concern.

It also said that private entities that make decisions on

that information are not state actors as long as they are not

threatened with adverse consequences. The department also said

that an injunction limiting the administration's actions could

chill vital government communications, including to protect

national security.

The plaintiffs sued officials and agencies across the

federal government, including in the White House, FBI, surgeon

general's office, Centers for Disease Control and Prevention and

the Cybersecurity and Infrastructure Security Agency.

Louisiana-based U.S. District Judge Terry Doughty issued a

preliminary injunction in July 2023. Doughty concluded that the

plaintiffs were likely to succeed on their claim that the

government helped suppress "disfavored conservative speech" on

mask-wearing, lockdowns and vaccines intended as public health

measures during the pandemic, or that questioned the validity of

the 2020 election in which Biden, a Democrat, defeated Donald

Trump, a Republican.

The injunction barred an array of government officials from

communicating with platforms regarding content moderation, such

as urging the deletion of certain posts.

The New Orleans-based 5th U.S. Circuit Court of Appeals

subsequently narrowed that order.

The Supreme Court's ruling is expected by the end of June.