WASHINGTON (AP) — A bipartisan drive to expand background checks is in deep trouble as the Senate approaches a long-awaited vote on the linchpin of the effort to curb gun violence.

The Senate planned to vote Wednesday on the proposal. As growing numbers of Republicans declared their opposition to the measure, supporters seemed headed toward defeat unless they can turn votes around in the closing hours.

The Senate will also vote on eight other amendments to a broad Democratic gun control bill.

The votes approached as an Associated Press-GfK poll shows ebbing support for tightening gun control laws.

The April survey finds 49 percent of Americans favor stricter gun curbs. That's down from 58 percent in January — just after the killings of children and aides at an elementary school in Newtown, Conn.