The Senate is debating S.649, the Safe Communities, Safe Schools Act, which would expand background checks on gun purchases, crack down on gun trafficking and beef up security in schools. GOP senators have vowed to block that bill, claiming it goes too far and infringes on the rights of gun-owners.

The bill doesn’t include an assault weapons ban or limits on magazine clip capacity — although Senate Majority Leader Harry ReidHarry ReidCabinet picks boost 2018 Dems Franken emerges as liberal force in hearings GOP eyes new push to break up California court MORE (D-Nev.) promised votes on those provisions as amendments.

The first amendment being considered is a bipartisan deal on background checks from Sens. Joe ManchinJoe ManchinOvernight Defense: McCain grills Trump's budget pick | Dems seek to limit Trump on nukes | Senators weigh new round of base closures Lawsuit filed to get information on Sessions Manchin to support Tillerson for State Department post MORE (D-W.Va.) and Pat Toomey (R-Pa.).

Republicans, including Grassley, have expressed concern that expanding background checks could create a federal registry of gun owners and make it harder for family members to transfer firearms. But Sen. Chuck SchumerCharles SchumerLive coverage: Trump budget chief faces two Senate panels McConnell to Dems: Work with us on GOP's 'formidable' challenges Democrats and the boycott of Trump's inauguration MORE (D-N.Y.) has insisted that background checks would be the “sweet spot” for both parties when it came to getting a deal on gun-control measures.

Manchin and Toomey’s deal would expand background checks to cover all sales at gun shows and over the Internet. Those background checks would have to be accompanied by records proving to law enforcement officials they took place. It would exempt gun sales and transfers between friends and acquaintances, and explicitly ban the federal government from creating a national firearms registry.

“We haven’t voted because despite claims from the other side, background checks have never been the ‘sweet spot’ in the gun-control debate,” Grassley said. “They don’t have the votes for background checks.”

Democrats have pointed to polls where nearly 90 percent of respondents support expanding background check requirements.

"If this is such an obvious question where 90 percent of Americans agree … why is this begin debated," Sen. Dick DurbinDick DurbinJustice requires higher standard than Sessions Warren burns Mnuchin over failure to disclose assets Trump Treasury pick to defend foreclosure record MORE (D-Ill.) said. "What is the big deal?"

Democrats would need 60 votes in order to clear the amendment, but so far only four Republicans have publicly stated their support for the Manchin-Toomey deal and it’s unclear that all 55 senators that caucus with the Democrats would support the amendment.

Grassley speculated that if the Senate moved to a different amendment first, it would be a sign that the majority is still searching for support to pass the Manchin-Toomey amendment.

“If we turn to assault weapons or magazines then it is clear to all that the majority is far from the number it needs,” Grassley said. “People are going to be waiting for time to pick up the votes that will never be there.”

Grassley said Republicans are eager to vote on amendments since his party has introduced several that strengthen gun owners’ rights.