KOENIGSWINTER, Germany (AP) — Treasury Secretary Janet Yellen celebrated a "historic day" last summer when more than 100 nations agreed to a global minimum tax deal, aimed at putting the world's countries on a more equal footing in attracting and keeping multinational companies. President Joe Biden tweeted that the idea was "diplomacy reshaping our global economy and delivering for our people."

But this week, as Yellen joined Group of Seven finance ministers for meetings in Germany, she found herself insisting that prospects for moving ahead with the landmark tax plan were merely "not hopeless."

The plan is running up against new resistance abroad and old divisions at hom e as fresh global concerns take center stage.

The ongoing war in Ukraine, the threat of rising food insecurity, crushing inflation and other urgent matters have stolen finance ministers attention away from putting the plan in place before a 2023 deadline. To add to the pressure, Poland solidified its opposition with a veto at an April European Union finance ministers meeting in Brussels. And Republicans in Congress are balking, too.

On Friday, the G-7 finance ministers wrapped up their two days of meetings with a joint statement that was most notable for announced pledges of $19.8 billion in economic aid for Ukraine. It included only brief mention of the tax idea, saying the ministers reiterated a "strong political commitment to the timely and effective implementation" of the plan to bring "new rules into effect at global level."

Broadly speaking, the global minimum tax deal is designed to subject large multinational firms to a 15% tax rate wherever they operate. The deal also provides for taxing part of the profits of the largest global companies in countries where they do business online but may have no physical presence.

It is supposed to halt an international race to the bottom for corporate taxation that has led multinational businesses to book their profits in countries with low tax rates. This enables them to avoid taxes and encourages countries to slash rates to attract the companies.

The G-7 website calls it "a genuine revolution in international tax law." French Finance Minister Bruno Le Maire has called it "the most important international tax agreement in a century."

But Poland is raising new concerns about how the plan would be implemented, and the G-7 meetings did not appear to break the logjam. EU rules require member nation unanimity to change tax-related laws.

Christian Lindner, Germany's finance minister, said at the end of the G-7 minister meetings that "all the technical concerns have been dispelled, so there can be no technical considerations any more, but … highly political ones."

Wydzial Prasowy, a spokesperson for Poland's finance ministry, cited concerns about the "lowering of the EU's competitiveness and placing of additional burden on European businesses" without ensuring that digital giants are adequately taxed. He added that the concerns were heightened "especially when faced with difficulties of the current post-pandemic times."

Yellen, who has made the tax deal one of her top priorities as Treasury secretary, opened this week's visit to Europe with a stop in Poland, in part to urge Polish leaders to reconsider their position.

"We're working to try to address their concerns," she told reporters Thursday. "We would love to see Poland come on board. I think it is not hopeless."

So far, 137 countries representing nearly 95 percent of the world's gross domestic product have agreed on the plan meant to "ensure that corporations fairly share the burden of financing government," she said.

But Yellen also faces headwinds at home from congressional Republicans who have displayed little appetite for having the United States hold up its end of the agreement. They say the plan would make the U.S. less competitive in a global economy.

Sen. Mike Crapo of Idaho, top Republican on the Senate Finance Committee, and Rep. Kevin Brady of Texas, the top Republican on the House Ways and Means Committee, both pointed to Poland's opposition in a joint statement last month.

"If the EU is already hitting roadblocks, no one should expect countries like China to implement this deal anytime soon," they said.

C. Eugene Steuerle, a fellow at the Urban Institute and a co-founder of the Urban-Brookings Tax Policy Center in Washington, said the deal may be unlucky to have been born in politically fractured times.

"What makes anything like this difficult these days is that the two parties are so divided," he said. "That's what really threatens this legislation more than the idea — which I think would traditionally have gotten support, at least some support from both sides of the aisle."

There's also just the crush of other global concerns demanding attention.

David Feldman, economics professor at the College of William & Mary in Virginia, says "governments have a certain amount of bandwidth — the current events have to push some of these other things a little further down the list."

Marc Goldwein, senior policy director of the private Committee for a Responsible Federal Budget, said the overall idea of the tax plan "is not to be punitive" but to "raise revenues for all nations."

"It also hopefully stops countries from cutting their taxes relative to other countries," he said.

According to the Congressional Research Service, since the mid-1960s, U.S. corporate tax payments have declined relative to the size of the economy — to roughly 1 percent of GDP in 2020 from 3.9 percent in 1965.