The Senate will take up a series of gun votes today, including the Manchin/Toomey proposal for extending background checks. As of Tuesday evening, however, gun-control advocates weren’t even sure they’d have enough votes to pass expanded background checks.  One Democratic aide told The Hill, “We don’t know if we have 60 votes, but we don’t know that we don’t have 60…it will be very close.”

The majority doesn’t have the votes to pass their own amendment, so we’re not voting,” said Sen. Chuck Grassley of Iowa, the ranking Republican on the Judiciary Committee. “If we turn to assault weapons or magazines, then it’s clear to all that the majority knows the votes aren’t going to be there.”

The Senate last week agreed to bring the gun bill to the chamber floor, with many Republicans supportive of that move. Since then, however, the bill has stalled and lawmakers have not voted on any amendments.

Earlier in the day, Democrats got a boost from the presence of former Rep. Gabrielle Giffords, Arizona Democrat, who was gravely wounded when a gunman opened fire at an outdoor town hall she was holding in Tucson in 2011. Democrats also heard an emotional plea for action from Sen. Joe Manchin III of West Virginia, a co-sponsor of the background check compromise, as well as Sens. Richard Blumenthal and Christopher Murphy of Connecticut and Tim Kaine of Virginia, who was governor of the state at the time of the Virginia Tech shootings in 2007.

The underlying bill has language on background checks that the compromise is supposed to replace.

For this reason, some pro-gun groups, such as the Citizens Committee for the Right to Keep and Bear Arms, are supportive of the Manchin/Toomey bill:

These advances for gun rights cannot be made unless we win the Senate vote to substitute the Manchin-Toomey language for Schumer’s invasive, terrible and overreaching background check that is in the current version of the bill.

If you read the proposed substitute bill, you can see the numerous advances for our Constitutional right to keep and bear arms that it contains: interstate sales of handguns, veteran gun rights restoration, travel with firearms protection, civil and criminal immunity lawsuit protection if you sell a gun plus more. It also exempts the sale or transfer of firearms between family members and friends as well as sales outside a commercial venue from a background check. If you have any kind of current state permit to own, use or carry no additional background check is done.

The NRA is not supportive of the proposal, however. And so far, only four of the 16 GOP senators that voted to bring the bill to the floor have expressed support or are leaning towards voting yes on the background check proposal: Sens. Toomey, Mark Kirk, Susan Collins and John McCain—though he’s not a definite yes.

Republican amendments could also make the background check bill all the more difficult to pass:

Mr. Grassley, Sen. Ted Cruz of Texas and other co-sponsors plan to unveil their substitute amendment Wednesday morning that makes changes to the National Instant Criminal Background Check System, provides resources to help address mental health and school safety, protects veterans from false health determinations, and addresses gun trafficking and straw purchasing.

In addition to expanded background checks, Senate Majority Leader Harry Reid has scheduled votes on at least eight other amendments, including bans on “assault weapons” and high-capacity magazines.