Sen. Joe Manchin (D-W.Va.) is telling colleagues he has deep concerns about a proposed “billionaire tax” but is waiting for more details before making a final decision, people familiar with the matter tell Axios.

Why it matters: The senator’s doubts reveal an uncomfortable truth for the White House and congressional leaders as they race to finish — and pay for — their nearly $2 trillion social spending and climate package: A tax solution designed to satisfy Sen. Kyrsten Sinema (D-Ariz.) isn’t necessarily acceptable to Manchin.

The billionaire tax gained currency over the weekend because Sinema, a fellow moderate and key negotiator, signaled her opposition to corporate, personal and capital gains tax hikes. Manchin’s potential opposition to the billionaire-tax proposal would blow a $200 billion to $250 billion hole in the Democrats’ plans for new revenues.