WASHINGTON >> In a highly anticipated announcement, the United States will offer a roughly 28 percent emissions cut as its contribution to a major global climate treaty nearing the final stages of negotiation, according to people briefed on the White House’s plans.

The U.S. plans to announce its commitment Tuesday, the informal deadline for nations to submit their contributions to the United Nations. Although the goal of 26 percent to 28 percent by 2025 isn’t new — President Barack Obama first unveiled it last year during a trip to Beijing — the U.S. proposal has drawn intense interest from the vast majority of countries that have yet to announce how deeply they’ll pledge to cut greenhouse gas emissions as part of the treaty.

Obama’s pledge constitutes the opening offer by the U.S. as world leaders strive to reach a climate deal powerful and ambitious enough to prevent the worst effects of climate change. In the works for years, the treaty is set to be finalized in Paris in December. If it’s successful, it will mark the first time all nations — not just wealthier ones like the U.S. — will have agreed to do something about climate change.

As part of its proposal, known to climate negotiators as an Intended Nationally Determined Contribution, the U.S. will also assert that its contribution is both ambitious and fair, said the individuals briefed on the U.S. proposal, who requested anonymity because the proposal hasn’t been announced.

What metrics the U.S. will use to back up that claim is not yet clear. The European Union, one of the first parties to submit its contribution, pointed to per capita reductions in emissions to show how it is cutting its carbon footprint. But emissions per capita are far higher in the U.S., making it an inconvenient measure for the U.S. to use to show progress.

Instead, the U.S. is expected to focus on the fact that the Obama administration has ramped up the rate of emissions reductions nearly twofold. Early in his presidency, Obama committed to cut U.S. emissions 17 percent by 2020; his subsequent goal for 2025 pushes it to 28 percent.

The White House declined to comment.

The U.S. and other developed countries have been aggressively pressing developing nations to step up on climate change — especially those like China and India that are heavily reliant on dirtier sources of energy. Obama has described his strategy as “leading by example” and has sought to use the steps he’s already taken to cut emissions to ramp up pressure on other countries to do the same.

But poorer countries have traditionally balked, arguing their more modest means make reductions more of an imposition and pointing out that historically, they’re responsible for just a small fraction of the heat-trapping gases that industrialized countries have been pumping into the atmosphere for decades. So when Obama and Chinese President Xi Jinping both committed to curbing emissions in a joint announcement in November, environmentalists hailed it as a sign that reluctant nations like China were finally getting on board.

“People know that domestically, we’re moving forward,” U.S. Environmental Protection Agency Administrator Gina McCarthy said Monday at a luncheon hosted by Politico. She pointed to the U.S.-China pact as Exhibit A. “If the two biggest polluters and the two biggest greenhouse gas polluters can get together, and two biggest economies, then we’re going to be OK moving into Paris, and we should have momentum behind our backs.”

Although all nations were asked to submit their climate targets by the end-of-March target date, only a handful of countries are expected to meet it. In addition to the U.S., the EU and Switzerland, Mexico unveiled a pledge last week to cut greenhouse gases and short-lived climate pollutants 25 percent by 2030, drawing praise from the White House and from environmental advocates.

How will the U.S. meet its goal? The Obama administration has avoided putting hard numbers on the size of emissions reductions it expects from specific steps the U.S. is taking. In its submission, the EU listed specific economic sectors — such as transportation, energy and manufacturing — where it expects major reductions, and named the specific greenhouse gases it plans to cut.

In contrast, the U.S. is expected to point broadly to the steps it is taking under the climate action plan Obama announced in 2013, such as new rules requiring sweeping cuts from new and existing power plants, stricter emissions limits for cars and trucks, and initiatives targeting specific greenhouse gases like methane and hydrofluorocarbons.

Many of those steps ordered by Obama face major legal challenges and intense political opposition, raising the risk that they could be undermined or even discarded once Obama leaves office in 2017. Contrast that with the EU, where lawmakers have approved a cap-and-trade emissions scheme that U.S. lawmakers have declined to enact.

“Just like every other country, this is a goal — it’s aspirational,” said Alden Meyer, policy director for the Union of Concerned Scientists. “It’s a fairly ambitious objective, especially if you assume that they’re not going to get any help from Congress.”