Corporate tax directors began the year trying to master the tax changes Congress enacted at the end of 2017, and they will start 2019 with a new challenge: navigating guidance the Treasury Department has written to explain the new law.

New regulations won’t be the only thing keeping tax officials up at night.

January 2019 brings the return of a divided Congress, after Democrats won back the House in the midterm elections. That likely means no substantial tax package will pass, leaving tax professionals to make the best of the status quo.

“There’s so much that we don’t know. I don’t know how many more spreadsheets I need to create,” said Myrtle Jones, senior vice president of tax at Halliburton Co., said Nov. 29 at the Bloomberg Tax Leadership Forum in Washington.

Here’s a look at the top issues corporate tax directors are facing next year:

Tax Reform Limbo

Companies are expecting final rules in late December or early 2019 on tax law changes such as Section 199A, the law’s 20 percent write-off for pass-through businesses. But the waiting period between proposed rules and final rules can leave companies uncertain about how to proceed if they still have questions.

“Some of the issues that are most important to us have not yet been addressed in proposed regulations,” the Securities Industry and Financial Markets Association said in a Nov. 15 letter to the Internal Revenue Service released under the Freedom of Information Act.

That means companies must decide whether to rely on proposed rules or to take a position that reflects what they think the final rules will—or should—say.

John Deere Co., for example, pushed the IRS for an exemption from the tax law’s levy on offshore cash, and said in a Sept. 21 letter that proposed regulations (REG-104226-18) “appear simply dismiss the issue.” The code section imposes a one-time tax on U.S. companies’ offshore earnings: 15.5 percent on cash and cash assets and 8 percent on illiquid assets. Companies can choose to pay the tax in installments over eight years.

An April agreement that gave the Office of Management and Budget’s Office of Regulatory and Information Affairs more authority to oversee tax regulations has added complexity.

“The traditional information pathways that companies use, especially large companies, of being able to call up Treasury and call people in the IRS and get a sense of what they’re thinking, has broken down because of the OMB,” said Andrew Silverman, a Bloomberg Intelligence analyst.

Calculating Net Interest Expense

Procter & Gamble’s main concern is the tax law’s levy on global intangible low-taxed income, said Timothy McDonald, vice president of global taxes.

Proposed rules (REG-106089-18) applying the law’s limit on debt interest payment write-offs to controlled foreign corporations—in which U.S. shareholders own more than 50 percent—came as an unwelcome surprise, he added. Calculating the net interest expense and deductibility of that interest for all of Procter & Gamble’s CFCs will be “a huge administrative headache,” McDonald told Bloomberg Tax.

The company has subsidiaries in dozens of countries, including Ireland and China, according to an annual filing from August.

The tax law capped deductions of debt interest expenses at 30 percent of adjusted taxable income, a limit that generally applies at the level of a consolidated group of companies that file as a single taxpayer, and to businesses with more than $25 million in annual gross receipts.

Digital Tax Proposals

A European Union effort to impose a 3 percent tax on large digital companies like Alphabet Inc.'s Google faltered over the course of the year.

France and Germany in December offered a scaled-back version aimed at European advertising revenue. A vote on that proposal is possible in March. It needs unanimous agreement from the EU member countries.

The Organization for Economic Cooperation and Development is also trying to find a solution that satisfies all its members, including the U.S. If it does so, the EU may drop its plan.

“What’s going to happen on digital, or as a result of digital, has everyone worried,” said Carol Doran Klein, vice president and international tax counsel at the U.S. Council for International Business.

Changing Global Tax Landscape

Ongoing international tax changes could have a profound effect on multinationals’ global operations.

“I’m convinced that in five years you won’t recognize the system we have in place” for global tax, Jeffrey Owens, director of Vienna University’s global tax policy center, said Dec. 3 at an event at Georgetown University Law Center in Washington.

Country-by-country reporting, in which multinationals must disclose financial data for each country in which they operate, has been widely adopted, and businesses fear they will be more vulnerable to audit.

Starting in January, companies will also begin seeing changes to the bilateral tax treaties that protect them from double taxation, as the OECD’s ambitious Multilateral Instrument, or MLI, goes into effect.

The MLI lets tax authorities modify many bilateral tax treaties instead of having to negotiate changes with each treaty partner. It will first apply to treaties between 15 countries, including the U.K. and Austria, and more countries will likely sign onto it in 2019.

Life After ‘Wayfair’

Small and medium-sized businesses with e-commerce activity are most concerned over what “home rule” states might do after the South Dakota v. Wayfair decision, said Harley Duncan, managing director and leader of the state and local tax group of the Washington National Tax practice of KPMG LLP.

The June decision tossed out Quill Corp. v. North Dakota, the Supreme Court’s 1992 physical presence standard for when states could tax remote sales.

Home rule states—Alabama, Colorado, and Louisiana—are states with self-administered local taxing authorities, such as cities or counties, that have chosen to collect and administer sales or use tax independently from the state.

“No home rule state has come out and said they will take this route, but if they did, the fear is that collection burdens would skyrocket through the roof,” Duncan said.