Global tax overhaul endorsed by 130 nations but obstacles remain.

The Biden administration and global allies scored a major victory Thursday in their push for a more balanced international corporate tax system, but still face multiple significant obstacles to completing an ambitious plan that has been years in the making.

The boost came during a round of talks hosted by the Organization for Economic Co-operation and Development, where 130 countries and jurisdictions backed a plan to set a minimum corporate tax rate and establish a new regime for sharing the taxes imposed on the profits of multinational firms.

Even as governments hailed the news — U.S. Treasury Secretary Janet Yellen called it “an historic day for economic diplomacy” — the agreement remains well short of a done deal. A handful of countries refused to sign on. Most important, resistance came from three European Union members, any of which could prevent the 27-member bloc from implementing the plan.

The U.S. Congress could also prove a major obstacle, since the legislature's approval would be required to formalize Washington's participation in the system, and President Joe Biden's Democratic party holds razor-thin majorities that are at risk in next year's midterm elections.

Thursday's deal also leaves several details in the proposals unresolved, including important questions over how and when some countries' unilateral taxes on tech firms' revenue will be rolled back.

All that casts next week's meeting of Group of 20 finance ministers in Venice in a new light, freeing officials to focus more on topics such as containing the COVID-19 pandemic rather than struggling to reach a deal on taxes. It could also represent a chance for senior officials to get on immediately with unfinished tax business.

“This will be the kickoff to the last stretch” leading up to the G-20 leaders' meeting scheduled for Oct. 30-31 in Rome, said Lilian Faulhaber, a law professor at Georgetown University who specializes in international tax issues.

“Venice gives them an opportunity to see exactly where are the areas of work that remain.”

The overhaul is aimed at helping countries share the spoils from multinational firms like Facebook Inc. and Alphabet Inc.'s Google, with implementation targeted for 2023. The rules would curtail tax avoidance by making global enterprises pay an effective rate of “at least 15 per cent” and give smaller countries more tax revenue from foreign firms.

Several key countries that had been question marks agreed to the terms, including India, China and Turkey, according to the OECD. In fact, Argentina and Turkey came on board at the last moment, according to officials familiar with the negotiations, and the technical details may leave room for further concessions to developing economies.

Most of the remaining holdouts can effectively be ignored without endangering the deal — except for EU members, because implementing the new rules there would require an EU directive, a legal act that must be approved unanimously by all 27 members.

That means Ireland and Hungary, which refused to back the plan, could effectively veto adoption in the EU, or at least force the bloc to resort to exceptional and untried legal measures for implementation.

A senior U.S. Treasury official acknowledged this represents a serious potential hurdle. Ultimately, the official said, for the new agreement to work, EU countries will need to be on board.

Ireland's finance minister, Paschal Donohoe, said Thursday that the nation isn't yet ready to agree to a global minimum tax rate of at least 15 per cent, though he intends to hold a public consultation in the country on the draft agreement, and the government pledged to “constructively engage” in the tax discussions.

French Finance Minister Bruno Le Maire said he would spend the coming week before the G-20 meeting redoubling his efforts to convince reluctant European countries to “make the necessary efforts to join an historic agreement that very broadly brings together the states of the planet.”

Europe isn't the only source of uncertainty. Congress will have to approve any legal changes necessary for the U.S. to comply with the agreement, and it appears clear that Republicans are opposed.

Kevin Brady of Texas, the senior Republican on the House Ways and Means Committee, issued a statement calling the OECD agreement a “dangerous economic surrender.”