JEFFERSON CITY, Mo. — Unless a handful of wavering Democrats change their minds, the Republican-controlled Missouri legislature is expected to enact a statute next month nullifying all federal gun laws in the state and making it a crime for federal agents to enforce them here. A Missourian arrested under federal firearm statutes would even be able to sue the arresting officer.

The law amounts to the most far-reaching states’ rights endeavor in the country, the far edge of a growing movement known as “nullification” in which a state defies federal power.

The Missouri Republican Party thinks linking guns to nullification works well, said Matt Wills, the party’s director of communications, thanks in part to the push by President Obama for tougher gun laws. “It’s probably one of the best states’ rights issues that the country’s got going right now,” he said.

The measure was vetoed last month by Gov. Jay Nixon, a Democrat, as unconstitutional. But when the legislature gathers again on Sept. 11, it will seek to override his veto, even though most experts say the courts will strike down the measure. Nearly every Republican and a dozen Democrats appear likely to vote for the override.