Instead of voting directly for presidential candidates, voters actually vote for electors appointed by each State, who then vote for the presidential candidate preferred by the voters. In 2016, however, three electors in the State of Washington pledged to vote according to what the voters preferred, but then violated that pledge and refused to vote for Hillary Clinton as they would have otherwise been required to, making them “faithless electors.” The State fined them $1,000 apiece. The electors challenged the fine, arguing that the Constitution gave them the right to vote how they wished, regardless of how the voters voted. The State supreme court affirmed the fines, and the Court unanimously held, in an opinion by Justice Kagan, that the fines were constitutional.
The majority held that the Constitution’s grant of authority to the States to appoint electors provided very broad power, including the power to punish those electors who did not act in accordance with their pledge. The majority also noted the historical role that electors played in the Electoral College which regularly limited their discretion and required them to vote as directed by others. Justice Thomas, joined in part by Justice Gorsuch, concurred in the result, arguing that the Constitution does not address elector voting, but that States enjoy the power to punish faithless electors through the Tenth Amendment.
A link to the opinion in Chiafalo v. Washington is here: https://www.supremecourt.gov/opinions/19pdf/19-465_i425.pdf