NUSA DUA, Indonesia — President Biden was in the Middle East and Treasury Secretary Janet L. Yellen was here in Southeast Asia when talks over the administration’s long-delayed sweeping economic agenda stalled yet again.Sen. Joe Manchin III (D-W.Va.), the Senate’s most conservative Democrat, communicated to Democratic leadership on Thursday that he would not support tax hikes on the wealthy or any new spending on clean energy as part of an economic package in Congress this summer, delivering a major blow to two of the administration’s most important policy goals.By Friday afternoon, Biden urged lawmakers to pass whatever they could to lower health-care costs, which Manchin has said he’ll support, and promised to take “strong executive action” to address climate change if the Senate wouldn’t act.But a president who spent 36 years in the Senate and another eight working closely with it as vice president had already taken a back seat in the attempt to push his economic agenda priority through a Congress his party controls by the narrowest of margins. The White House had much earlier left Senate Majority Leader Charles E. Schumer (D-N.Y.) to negotiate with Manchin over how to assemble a bill the West Virginia centrist could support. White House Chief of Staff Ron Klain has long been skeptical about the potential for a deal with Manchin, according to three people with direct knowledge of his private remarks.