Updated 5:55 p.m. Tuesday: Sufficient support to ensure final passage of a bipartisan plan to extend the gun background check program to include most commercial sales is at risk, based on the latest vote count.

Sens. Patrick Toomey (R-Pa.) and Joe Manchin III (D-W. Va.) appeared Sunday on CBS's "Face the Nation." (AP)

The amendment will require at least 60 votes to be added to the underlying gun bill.

Based on analysis by The Washington Post, only 22 of the 100 Senate votes are in play: The 16 Republican senators who voted last week to proceed with debate and six moderate Democrats who face difficult reelections in 2014 or represent rural states with strong gun cultures and would face strong political pressure at home for supporting new gun-control legislation.

Based on our analysis, here's where things stand as of 5:55 p.m. Tuesday:

Yes: 52

No: 40

Undecided: 7 (Ayotte, Baucus, Begich, Heitkamp, Landrieu, McCain, Pryor)

Fate unclear: 1 (Lautenberg)

** The 16 Republicans who voted with Democrats to proceed: Lamar Alexander (R-Tenn.), Kelly Ayotte (R-N.H.), Richard Burr (R-N.C.), Saxby Chambliss (R-Ga.), Tom Coburn (R-Okla.), Susan Collins (R-Maine), Bob Corker (R-Tenn.), Jeff Flake (R-Ariz.), Lindsey Graham (R-S.C.), Dean Heller (R-Nev.), John Hoeven (R-N.D.), Johnny Isakson (R-Ga.), Mark Kirk (R-Ill.), John McCain (R-Ariz.), Patrick Toomey (R-Pa.) and Roger Wicker (R-Miss.).

** The 11 Senate Republicans who voted to proceed and now plan to vote no on the Manchin-Toomey amendment: Alexander, Burr, Chambliss, Coburn, Corker, Flake, Graham, Heller, Hoeven, Isakson, Wicker.

** Members of the "Sweet 16" who plan to vote yes for the Manchin-Toomey amendment: Collins, Kirk, Toomey.

** Still undecided: Ayotte, McCain.

** The six at-risk Senate Democrats: Max Baucus (Mont.), Mark Begich (Alaska), Kay Hagan (N.C.), Heidi Heitkamp (N.D.), Mary Landrieu (La.), Mark Pryor (Ark.).

** Democrats who plan to vote no on Manchin-Toomey: None have publicly declared they're a "no" vote.

** Democrats who plan to vote yes on Manchin-Toomey: Hagan

** Democrats still undecided: Baucus, Begich, Heitkamp, Landrieu, Pryor. (Spokesmen for the senators said over the weekend that they continue to review the proposal.)

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