By Timothy Aeppel and Daniel Wiessner

NEW YORK (Reuters) - Unions in the United States face sweeping changes to labor law and regulations under a new Republican administration that is expected to tilt policy toward employers.

Republicans will take control of the presidency and both chambers of Congress in Tuesday’s elections, after unions failed to deliver key industrial states to Democratic candidate Hillary Clinton.

Clinton's loss could have wide-ranging consequences for unions. Donald Trump and a Republican-led Senate will likely pick the next justice for the Supreme Court, which often hears labor-related cases.

”I think it’s going to be a very difficult period,” said Lee Saunders, president of the American Federation of State, County and Municipal Employees, who said he was surprised by the election outcome despite knocking on doors and talking to members in several states.

Among his concerns, he listed a Supreme Court case this year in which public-sector unions scored a victory related to funding organized labor – but only because the court deadlocked 4-4. The appointment of a new conservative judge by Trump to replace the late Justice Antonin Scalia could change that.

Republican nominees will control the National Labor Relations Board (NLRB), which under Democratic President Barack Obama has sought to make it easier for unions to organize. The NLRB is pursuing litigation to establish that McDonald's Corp is a "joint employer" of workers at its franchised restaurants, a determination that could compel the company to bargain with unions representing employees of its independent franchisees.

Republicans will control federal agencies that govern the formation of unions, overtime rules and more.

For example, in September the Obama administration finalized an executive order requiring federal contractors to provide sick leave to workers, as well as rules expanding the types of data employers are required to provide on pay. A separate Labor Department rule expanding which employees are eligible for overtime pay is scheduled to take effect next month.

Those actions drew criticism from business groups, and all could be reversed under a Trump administration.

“We don’t have a firewall now,” said Tom Buffenbarger, a prior president of the International Association of Machinists and Aerospace Workers who campaigned for Clinton.

Trump has expressed support for so-called right-to-work legislation, which allows workers to avoid paying union dues. Republican leaders in Congress have consistently sought such a change at the national level. Labor unions say such laws are aimed at undermining collective bargaining and workers’ rights.

William Gould, chair of the NLRB under Democratic President Bill Clinton and now a professor at Stanford Law School, said on Wednesday that Trump was likely to partner with Congress in dismantling a host of administration labor initiatives from Obama.

Republicans “regard unions as first amongst fair game” because of their support for Clinton, he said.

Steven Bernstein, a partner at law firm Fisher Phillips, which represents employers, said the Trump administration and Congress may also target recent NLRB rulings that allowed workers to picket on private property, expanded the type of worker activity protected by federal labor law and gave graduate students the right to unionize.

“It's also fair to assume that Trump will be inclined to repeal a host of executive orders supporting unions," particularly rules that apply to federal contracts, Bernstein said in a statement.

Organized labor is a key Democratic constituency – contributing everything from volunteers to votes for Democratic candidates. Trump nonetheless carried union-heavy states such as Pennsylvania, Ohio and even Michigan, home to the United Auto Workers (UAW).

In Ohio, for example, a Fox exit poll said Trump won 52 percent of union workers. An exit poll from ABC News suggested Clinton got no advantage among union households, even after Obama won those voters by 23 percentage points. (http://abcn.ws/2eUMxpx) (http://fxn.ws/2eULxS8)

David Lephart, a 58-year-old UAW pipefitter at a General Motors Co plant in Marion, Indiana, voted for Trump. He earns about $100,000 a year, with overtime, he says, and has put both of his kids through college.

He said union leaders pressed hard for members to support Clinton, but the union has nOt kept its promises to workers in recent years. “I don’t have a tendency to believe what you tell me — when you haven’t fulfilled the promises you’ve made,” he said.

UAW President Dennis Williams in a statement on Wednesday said, "we have high hopes that elected officials heard the American people loud and clear about trade, jobs, education and the inequality in this country."

(Writing by Luciana Lopez in Miami; Additional reporting by Jeffrey Dastin in New York; Editing by Joseph White and Jonathan Oatis)