WASHINGTON — A Democratic senator vital to the fate of President Joe Biden’s $3.5 trillion plan for social and environmental spending said Sunday he won’t support even half that amount or the ambitious timetable envisioned for passing it.

The stand by Sen. Joe Manchin, D-W.Va., was described as unacceptable by the chairman of the Senate Budget Committee, who is helping craft the measure. But Democrats have no votes to spare if they want to enact Biden’s massive “Build Back Better” agenda, with the Senate split 50-50 and Vice President Kamala Harris the tiebreaker if there is no Republican support.

With congressional committees working toward the target of Wednesday set by party leaders to have the bill drafted, Manchin made clear his view, in a series of television interviews, that there was “no way” Congress would meet the late September goal from House Speaker Nancy Pelosi, D-Calif., for passage.

“I cannot support $3.5 trillion,” Manchin said, citing in particular his opposition to a proposed increase in the corporate tax rate from 21% to 28% and vast new social spending.

“We should be looking at everything, and we’re not. We don’t have the need to rush into this and get it done within one week because there’s some deadline we’re meeting, or someone’s going to fall through the cracks,” he said.

Pressed repeatedly about a total he could support, Manchin said, “It’s going to be $1, $1.5 (trillion).” He later suggested the range was based on a modest rise in the corporate tax rate to 25%, a figure he believes will keep the U.S. globally competitive.

“The numbers that they’re wanting to pay for and the tax changes they want to make, is that competitive?” Manchin asked. “I believe there’s some changes made that does not keep us competitive.”