The two decisions were the latest in a stunning run of victories for a conservative agenda that has increasingly been built on the foundation of free speech. Conservative groups, borrowing and building on arguments developed by liberals, have used the First Amendment to justify unlimited campaign spending, discrimination against gay couples and attacks on the regulation of tobacco, pharmaceuticals and guns.

“The right, which had for years been hostile to and very nervous about a strong First Amendment, has rediscovered it,” said Burt Neuborne, a law professor at New York University.

The Citizens United campaign finance case, for instance, was decided on free-speech grounds, with the five-justice conservative majority ruling that the First Amendment protects unlimited campaign spending by corporations. The government, the majority said, has no business regulating political speech.

The dissenters responded that the First Amendment did not require allowing corporate money to flood the political marketplace and corrupt democracy.

“The libertarian position has become dominant on the right on First Amendment issues,” said Ilya Shapiro, a lawyer with the Cato Institute. “It simply means that we should be skeptical of government attempts to regulate speech. That used to be an uncontroversial and nonideological point. What’s now being called the libertarian position on speech was in the 1960s the liberal position on speech.”