© David Kasnic for The Wall Street Journal
Denny Peyman is one of a group of sheriffs who say they won't enforce gun laws they deem unconstitutional.
A collection of sheriffs across the country have sent Congress and the Obama administration a message: If we don't like your gun laws, we aren't going to help enforce them.

As of Thursday, 90 sheriffs, many from rural counties, had pledged not to enforce laws they deem unconstitutional, according to a list compiled by the Constitutional Sheriffs and Peace Officers Association, a Texas-based group that has "vowed to uphold and defend the Constitution against Obama's unlawful gun control measures."

A White House spokesman declined to comment.

The sheriffs began their campaign of letters and media interviews earlier this month, shortly after President Barack Obama signed 23 executive orders addressing gun violence, and outlined a broad framework of new gun-control measures for Congress to consider.

Among the more controversial proposals from the president: a ban on the sale of certain semiautomatic rifles and high-capacity ammunition magazines. No one has proposed a measure requiring the confiscation of these weapons or magazines, a scenario that several sheriffs interviewed said were among their chief fears.

Sen. Dianne Feinstein (D. Calif.) and other Democratic lawmakers unveiled legislation Thursday that would ban the sale of 158 types of weapons, as well as magazines holding more than 10 rounds of ammunition.

It is unlikely that the sheriffs, who represent just 3% of the country's total, will prove a major obstacle to any laws Congress ultimately passes in the wake of the elementary-school shootings last month in Newtown, Conn. But concerted efforts to thwart new laws could hinder federal investigations and prosecutions, say law-enforcement experts. And the fact that a group of sheriffs are protesting in a public way is a significant departure for law-enforcement officials.

Federal agents rely on local law enforcement to tip them off to possible federal crimes and they work together on criminal investigations. Mike Bouchard, a former assistant director at the Bureau of Alcohol, Tobacco, Firearms and Explosives who oversaw the agency's field operations, said the ATF relies on local law-enforcement officials to alert the agency when they stop someone who has a banned weapon, for example.

"If state and locals choose not to contribute, it would have a major impact," Mr. Bouchard said.

Larry Amerson, sheriff of Calhoun County, Ala., and president of the National Sheriffs' Association, the largest professional organization of sheriffs, said the group supports Mr. Obama's executive actions, which include improving incentives for states to share information with the background-check system and directing the Centers for Disease Control and Prevention to research gun violence. The National Sheriffs' Association hasn't taken a position on the legislation unveiled Thursday, but it will be discussed at the group's winter conference in Washington next week, he said.

Sheriff Amerson said concerns about weapon confiscation were overblown. "We're very comfortable that laws of that nature are not going to pass and we would oppose them strongly," he said.

Law-enforcement groups including the International Association of Chiefs of Police have largely supported gun-control efforts through the years, including bans on semiautomatic weapons.

Jackson County, Ky., Sheriff Denny Peyman was among the first to complain publicly about the White House proposals. Even modest measures to restrict guns are a threat to his constituents' rights, he said. "What they're doing is they're trying to chip away at the Constitution, basically crack the door," he said

The letters come as state legislators in Alaska, Kansas, Tennessee, Utah and Wyoming have proposed bills to pre-empt any forthcoming federal gun restrictions. The state proposals include making it a crime for federal agents to enforce federal restrictions and carving out states as exempt from restrictions altogether. The bills are a largely symbolic gesture. When a federal law conflicts with a state law, the federal law prevails, under the Supremacy Clause of the U.S. Constitution.

At least one sheriff, J. Grayson Robinson in Arapahoe County, Colo., which includes the town of Aurora, where 12 people were killed last year in a shooting at a movie theater, has questioned his colleagues' efforts. On Tuesday, Mr. Robinson called "ill-conceived" the idea that law-enforcement officials can carry out policies based on their own reading of the Constitution.

The opposition to the latest gun-control efforts have some precedent. Richard Mack, former sheriff in Graham County, Ariz., and Jay Printz, a former Montana sheriff who now sits on the board of the NRA, sued the federal government in 1994 to block a provision of the Brady Handgun Violence Prevention Act that required local law enforcement to conduct background checks on gun purchasers while the federal government established its own system.

Three years later, the Supreme Court ruled that the federal government couldn't compel states to carry out the task, though many states had voluntarily agreed to do so. A year later, the Federal Bureau of Investigation launched National Instant Criminal Background Check System, which licensed gun dealers must consult when making sales.

Kristin Goss, a professor of public policy and political science at Duke University, said there is a historical schism between individual sheriffs in rural areas and their national association, which has backed all major gun-control laws since the 1990s. Some sheriffs' views on guns largely mirror those held by constituents who harbor a distrust of the federal government, she said.

Douglas County, Ore., sheriff John Hanlin, one of those vowing not to enforce laws they deem unconstitutional, expressed his views in a letter to Vice President Joe Biden, who led a task force to study legislative avenues after the shootings at Sandy Hook Elementary School. "Let's put it this way," he said. "I'm sure not going to bend over backwards to take away weapons, especially if they were purchased legally by law-abiding and responsible citizens."