Until 2010, federal law barred companies from using corporate money to endorse and campaign for political candidates — and that included urging employees to support specific politicians.

But the Supreme Court's Citizens United decision has freed companies from those restrictions, and now several major companies, including Georgia-Pacific and Cintas, have sent letters or information packets to their employees suggesting — and sometimes explicitly recommending — how they should vote this fall.

In these letters, the executives complain about the costs of overregulation, the health care overhaul and possible tax increases. Some letters warn that if President Obama is re-elected, the company could be harmed, potentially jeopardizing jobs.

David A. Siegel, 77, chief executive of Westgate Resorts, a major time-share company, wrote to his 7,000 employees, saying that if Mr. Obama won, the prospect of higher taxes could hurt the company's future.

"The economy doesn't currently pose a threat to your job. What does threaten your job, however, is another four years of the same presidential administration," Mr. Siegel wrote. "If any new taxes are levied on me, or my company, as our current president plans, I will have no choice but to reduce the size of this company."

In an interview, Mr. Siegel said he was not ordering his employees to vote his way. "There's no way I can pressure anybody," he said. "I'm not in the voting booth with them."

Mr. Siegel added: "I really wanted them to know how I felt four more years under President Obama was going to affect them. It would be no different from telling your children: 'Eat your spinach. It's good for you.' "

Dave Robertson, the president of Koch Industries, sent an information packet and letter this month to more than 30,000 employees of a subsidiary, Georgia-Pacific, a paper and pulp company. The letter attacked government subsidies for "a few favored cronies" as well as "unprecedented regulatory burdens on businesses."

The letter added, "Many of our more than 50,000 U.S. employees and contractors may suffer the consequences, including higher gasoline prices, runaway inflation and other ills."

The Georgia-Pacific letter, first reported by In These Times, included a flier listing several candidates endorsed by the Koch brothers, the conservative billionaires, beginning with Mitt Romney, as well as opinion articles that the brothers had written.

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Travis McKinney, a forklift driver for Georgia-Pacific in Portland, Ore., said the company's political packet had spurred widespread discussion. "It leaves a bad taste," Mr. McKinney said. "I won't even wear my Obama pin to work because of the mailer."

In a statement, Koch Industries said its mailing contained pieces of information "we believe are important for our employees to know about." The company said the letter was in no way intimidation: "We make it clear that any decision about which candidates to support belongs solely to our employees."

Other companies whose top executives have sent out anti-Obama letters include Rite-Hite, a manufacturer of industrial equipment based in Milwaukee, and ASG Software Solutions, based in Naples, Fla.

Many corporate executives say they have stepped up their political activities to counter organized labor's efforts on behalf of Mr. Obama and other Democrats. Even before Citizens United, unions were allowed to promote candidates to their members. Democrats and Republicans alike acknowledged the effectiveness of labor's political efforts.

Mr. Romney has himself urged business owners to appeal to their employees. In a conference call in June organized by the National Federation of Independent Business, he said, "I hope you make it very clear to your employees what you believe is in the best interest of your enterprise and therefore their job and their future in the upcoming elections."

Larry Gold, associate general counsel of the A.F.L.-C.I.O., said some of the recent employer letters, by hinting at the possible loss of employees' jobs, appeared to cross the line into improper coercion. Federal law and the laws of several states bar anyone from coercing or intimidating voters into voting a certain way.

But Bradley A. Smith, a Republican former member of the Federal Election Commission and a professor at Capital University Law School, disagreed, saying letters like those sent by the companies were not firm threats to fire anyone if Mr. Obama won.

According to the Citizens United ruling, companies may recommend candidates to employees, said Eugene Volokh, a law professor at University of California, Los Angeles.

"If the employer wants to say, 'This candidate is good or bad for our business and therefore good or bad for you, the employee, that's permissible — that's protected by the First Amendment," Professor Volokh said. "But if the employer threatens to fire you based on how you vote, that's not protected."

But many liberal legal experts fear that employees could be discouraged from exercising their rights to free speech.

"The concern here is there is an unavoidable power disparity between management and employees," said Adam Skaggs, senior counsel at the liberal Brennan Center for Justice. "Put yourself in the shoes of an employee at any of those companies. Are you going to be comfortable putting an Obama bumper sticker on your car and driving into the company parking lot? If you're in a small community with a big employer, will you feel uncomfortable about putting up a yard sign for a candidate your boss doesn't favor?"

Richard Lacks, chief executive of Lacks Enterprises, an auto parts company based in Grand Rapids, Mich., wrote to his 2,300 employees this month warning that an Obama victory would mean higher health care costs and higher taxes that would eat into their paychecks. "It is important that in November you vote to improve your standard of living and that will be through smaller government and less government," he wrote.

Scott D. Farmer, chief executive of Cintas, the uniform supply company, sent a letter to his company's 30,000 employees on Oct. 19, denouncing the Affordable Care Act and saying it "amounts to the single largest tax on Americans and business in history." He warned employees that "the overregulation that business is facing today from the various administrative agencies such as the Environmental Protection Agency" and the National Labor Relations Board "is suffocating many companies."

Mr. Farmer added, "This uncertainty felt by many of our customers about their ability to run and grow their businesses prevents them from adding jobs which hurts our ability to grow and add jobs."

Asked about Mr. Farmer's letter, Greg Hart, Cintas's vice president for government affairs, responded, "The communication was not an attempt to suggest to employees how to vote, but rather it was sent to help partners make an informed decision."

Election law experts did not point to any corporate efforts this year to urge employees to back Mr. Obama, although corporations have at times politicked for Democratic candidates. In 2010, Harrah's, the casino company, urged its employees to go to the polls to re-elect Nevada's senior senator, contending that "waking up to the defeat of Harry Reid Nov. 3 will be devastating."