WASHINGTON — On the final day of the Supreme Court term last week, Justice Elena Kagan sounded an alarm.

The court’s five conservative members, citing the First Amendment, had just dealt public unions a devastating blow. The day before, the same majority had used the First Amendment to reject a California law requiring religiously oriented “crisis pregnancy centers” to provide women with information about abortion.

Conservatives, said Justice Kagan, who is part of the court’s four-member liberal wing, were “weaponizing the First Amendment.”

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.