This one is a labor / First Amendment crossover. A union claimed Idaho law, prohibiting deductions from employees' pay for union political action funds, is unconstitutional in violation of the First Amendment. The Court disagreed and upheld Idaho law.
Here's the issue and the Court's resolution, as framed by the Court itself:
Under Idaho law, a public employee may elect to have a portion of his wages
deducted by his employer and remitted to his union to pay union dues. He may not, however, choose to have an amount deducted and remitted to the union’s political action committee, because Idaho law prohibits payroll deductions for political activities. A group of unions representing Idaho public employees challenged this limitation. They conceded that the limitation was valid as applied at the state level, but argued that it violated their First Amendment rights when applied to county, municipal, school district, and other local public employers.
We do not agree. The First Amendment prohibits government from “abridging the freedom of speech”; it does not confer an affirmative right to use government payroll mechanisms for the purpose of obtaining funds for expression. Idaho’s law does not restrict political speech, but rather declines to promote that speech by allowing public employee checkoffs for political activities. Such a decision is reasonable in light of the State’s interest in avoiding the appearance that carrying out the public’s business is tainted by partisan political activity. That interest extends to government at the local as well as state level, and nothing in the First Amendment prevents a State from determining that its political subdivisions may not provide payroll deductions for political activities.
So, there you have it. The case is Ysursa v. Pocatello Ed. Assn. and the opinion is here.