"Seditious Conspiracy" Is Still a Crime
But unlike the Sedition Act of 1798, the federal seditious conspiracy statute doesn't focus on antigovernment speech (such as alleged lies about the government).
Some comments on the Michigan AG's #DetroitLeaks Takedown Demand, and Seditious Libel post reminded me to write about this: Occasionally, we hear about calls to prosecute people for "seditious conspiracy," but this means
conspir[ing] to overthrow, put down, or to destroy by force the Government of the United States, or to levy war against them, or to oppose by force the authority thereof, or by force to prevent, hinder, or delay the execution of any law of the United States, or by force to seize, take, or possess any property of the United States contrary to the authority thereof.
This is just a special case of the broader proposition that conspiring to commit a crime can itself be a crime. You can be punished under state law for conspiring to commit murder or theft or what have you. You can be punished under federal law for conspiring to commit bank robbery, or to defraud the federal government. Likewise, you can be punished under the "seditious conspiracy" statute for conspiring to illegally oppose the enforcement of the law.
This is quite a different statute from the Sedition Act of 1798 (or from the common-law crime of seditious libel), which punished (among other things) false and malicious speech intended to defame the federal government. And to the extent that the seditious conspiracy law punishes agreements to commit crime, which may be expressed by speech, such conspiracy is viewed as constitutionally unprotected, because it is speech integral to the criminal conduct that is being planned. For more on this, see U.S. v. Rahman (2d Cir. 1999).