Washington (CNN) -- A compromise proposal to expand background checks on gun sales lacks enough support to win Senate approval now, which will likely delay a vote on the measure that has strong public support, one of the sponsors told CNN on Monday. Sen. Joe Manchin, D-West Virginia, acknowledged the vote on the amendment he worked out with Republican Sen. Pat Toomey of Pennsylvania was being pushed back to try to build more support. Asked if the vote would take place Tuesday or Wednesday, as supporters had hoped, Manchin said he didn't think so. "I would say by the end of the week, probably," he added.
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NEW: Sen. Manchin says a vote on background checks unlikely until later in the week
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A compromise amendment on background checks comes up this week
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Republicans who voted to open debate may not support the bill
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Debate on the overall gun package is expected to last two weeks
So far, four GOP senators have declared they will support the @placeholder-Toomey compromise or were leaning toward backing it.