DENVER — The same day that President Obama unveiled his plan to address gun violence, the sheriff of Larimer County, Justin Smith, took to his personal Facebook page.

In an impassioned post, Sheriff Smith, whose jurisdiction includes a swath of suburbia and farmland north of Denver, railed against universal background checks and wrote of the need to defend the Second Amendment.

“As Sheriff, I will not enforce unconstitutional federal laws,” he wrote.

Over the past several weeks, dozens of other sheriffs from across the country have reacted with similar public opposition to Mr. Obama’s call for stiffer gun laws, releasing a deluge of letters, position papers and statements laying out their arguments in stark terms. Their jurisdictions largely include rural areas, and stand in sharp contrast to those of urban police chiefs, who have historically supported tougher gun regulations.

“I don’t plan on helping or assisting with any of the federal gun laws because I have the U.S. Supreme Court and the U.S. Constitution on my side,” said Sheriff John Cooke of Weld County, Colo. Sheriff Cooke said that he believed a ban on assault weapons would do little and that universal background checks would unfairly halt private gun sales.