WASHINGTON (AP) — 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. [...] poorer countries have 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. 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. 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. [...] the U.S.