WASHINGTON—Two centrist senators key to negotiations over infrastructure spending signaled progress in talks but made clear they don’t support President Biden’s expansive $2.3 trillion proposal.

Sen. Joe Manchin (D., W.Va.) said Sunday he wants the focus to be on conventional infrastructure like highways and bridges and that spending should be split off from broader items in Mr. Biden’s proposal, such as $400 billion to help care for the aging and those with disabilities.

“I do think they should be separated. Because if you start putting so much into one bill, which we call an omnibus bill, it makes it very, very difficult for the public to understand,” Mr. Manchin said on CNN.

Republican Sen. Shelley Moore Capito, also from West Virginia, said she had received positive feedback from the White House after she helped craft a $568 billion infrastructure proposal, which was cast as a starting point for bipartisan negotiations.

“This is an active conversation and I think that it’s a good beginning,” she said on CNN of the plan, which doesn’t provide specifics how to fund the investments and some Democrats say is far too limited.