Updated at 3:17 p.m.

Sens. Kelly Ayotte (R-N.H.), Heidi Heitkamp (D-N.D.) and Mark Pryor (D-Ark.) announced Wednesday that they will vote against the gun control compromise spearheaded by Sens. Joe Manchin (D-W.Va.) and Pat Toomey (R-Pa.), all but assuring the measure won't pass.

Ayotte's office confirmed Wednesday morning that she will not vote for the amendment, making her what appears to be the 41st vote against it. Later, Heitkamp and Pryor became the first Democrats to come out publicly against it.

"I commend Sens. Manchin and Toomey for working so hard to bring a serious bill to the floor," Heitkamp said. "However, in its current form I do not see a path for my support. I’ve thought long and hard about this, I’ve taken the tough meetings, and I’ve heard overwhelmingly from the people of North Dakota; and at the end of the day my duty is to listen to and represent the people of North Dakota.”

Ayotte, Pryor and Heitkamp now make 43 senators who are opposed to the amendment, which means there aren't enough votes to reach the 60-vote threshold it needs for passage.

In related news, Sens. John McCain (R-Ariz.) on Wednesday afternoon became the fourth Republican to support the deal, and Sen. Frank Lautenberg (D-N.J.) will return to the Senate chamber for the gun control votes despite being in poor health. There had been some question about whether he would be able to return if Democrats needed him for the 60th vote.

Below is the updated whip count:

Yes: 54

No: 43

Undecided: 3 (Baucus, Begich, Landrieu)

** 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 12 Senate Republicans who voted to proceed and now plan to vote no on the Manchin-Toomey amendment: Alexander, Ayotte, 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, 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: Heitkamp, Pryor.

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

** Democrats still undecided: Baucus, Begich, Landrieu.