ABC News' Sunlen Miller and Jeff Zeleny report:

The day of reckoning is Wednesday for the embattled Manchin-Toomey background check provision and a myriad of other gun amendments, including a ban on assault weapons and high-capacity clips.

The outcome of which will determine the fate of the biggest gun control legislation the Senate will vote on in two decades.

A 4 p.m. vote on the Manchin-Toomey amendment will kick off the votes.

The amendment, proposed this past week as a bipartisan compromise from Sen. Joe Manchin, a Democrat from West Virginia, and Sen. Pat Toomey, a Republican from Pennsylvania, always faced an uphill climb to pass in the Senate.

But the first real signs of trouble came Monday when a vote on the amendment was delayed from being formally scheduled when it was clear that the votes were not yet there for it to pass. By Tuesday, momentum seemed to slip away bit-by-bit when a few senators key to the outcome of the vote, including Sen. Dean Heller, R-Nev., Sen. John Hoeven, R-N.D., and Sen. Richard Burr, R-N.C., announced that they could not vote for the bill.

The amendment will need 60 votes to pass. And as of tonight, the votes are not there yet.

When Manchin was asked by ABC News if he had 60 votes locked down, he said: "We need more than we have."

Sen. Mark Kirk of Illinois, one of the Republican supporters for expanding background checks, said he was still working to win over some Republican senators. When asked if his side had enough votes to pass the amendment, he said: "We are not ready for a vote."

The vote will be razor thin - so thin that neither side was sounding confident.

There are three Republican senators and four Democratic senators believed still undecided - John McCain, R-Ariz., Lisa Murkowski, R-Alaska, Kelly Ayotte, R-N.H., Mary Landrieu, D-La., Mark Begich D-Alaska, Heidi Heitkamp, D-N.D., and Mark Pryor, D-Ark.

Sen. Frank Lautenberg, D-N.J., was seen as a wild card because, although he supports the amendment, he has been ill and home in New Jersey. Aides said Lautenberg "hopes" to get back for the vote Wednesday.

Senate Majority Leader Harry Reid, D-Nev., sounded a bit resigned Tuesday when he defended the bill's momentum while, in the same breath, admitting that the votes may not be there. Regardless, he said, gun control supporters have the "wind at our back."

President Obama made calls to the few undecided senators Tuesday, ABC News' Jonathan Karl reported. A White House official said there still was a path to 60 votes but conceded it is "a narrow path."

Yet the situation remained fluid, Republican and Democratic aides told ABC News, and either outcome was possible when the voting was to begin at 4 p.m. on Wednesday.

Following the Manchin-Toomey amendment vote, the Senate will vote on at least eight other gun amendments, all of which matter to the debate. They included voting up or down on an assault weapons ban, the issue of concealed carry, a high-capacity clip ban and mental health provisions.