Senators will take up the bipartisan measure from Joe Manchin (center) and Pat Toomey. Senate to vote on guns Wednesday

The Senate will vote Wednesday afternoon on what could be the biggest changes in U.S. gun laws in nearly 20 years.

Senators will take up the proposal from Sens. Joe Manchin (D-W.Va.) and Pat Toomey (R-Pa.) to expand background checks on firearm purchases and close the so-called gun show loophole. The bipartisan plan is likely the strongest gun control bill that can pass this Congress, although it’s far weaker than the White House and many Democrats hoped for.


Nevertheless, it will require 60 votes to pass, and the White House and Democrats still lack the final votes they need.

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Voting on the Manchin-Toomey propsal is scheduled to begin at 4 p.m., and both opponents and supporters say the margin is razor thin at this point.

Even after nine votes on amendments, the Senate will not have finished work on a gun bill. A vote on final passage of whatever gun bill remains standing has not yet been set. Both Democrats and Republicans want to see how Wednesday’s showdown over one of President Barack Obama’s top legislative priorities plays out before making any decisions on whether they will let the underlying gun package move forward.

Senate Majority Leader Harry Reid and his GOP counterpart, Minority Leader Mitch McConnell, finalized the agreement on Tuesday night setting up votes on eight additional amendments. Those amendments will also require 60 votes to pass.

They include Democratic proposals such as a ban on assault weapons, a prohibition on high-capacity ammunition magazines, and a measure to expand mental-health screening.

GOP leaders, vehemently opposed to the Manchin-Toomey plan have submitted their own proposals, including an alternative gun bill by Sen. Chuck Grassley (R-Iowa); an amendment from provision by Sen. John Cornyn (R-Texas) to grant “reciprocity” in other states for gun owners with “concealed carry permit; a proposal by Sen. Richard Burr (R-N.C.) that would prevent veterans deemed “mentally defective” from automatically being barred from buying guns; and a measure by Sen. John Barrasso (R-Wyo.) related to privacy of gun owners.

( PHOTOS: Politicians speak out on gun control)

A bipartisan proposal by Judiciary Committee Patrick Leahy (D-Vt.) and Sen. Susan Collins (R-Maine) to increase penalties for gun trafficking is expected to be approved by voice vote.

But for Democrats, Manchin-Toomey is the big one. Vice President Joe Biden has spoken with several senators, and former Rep. Gabby Giffords (D-Ariz.), who was seriously wounded in a January. 2011 shooting that killed six people, that left six other people dead, was on Capitol Hill on Tuesday to press for the Manchin-Toomey plan.

Giffords received a long ovation when she entered the Senate Democratic Caucus luncheon with her husband, Mark Kelly.

“She’s our whip,” Manchin declared.

Giffords and Kelly also attended a ceremony dedicating a room in the Capitol Visitor Center to Gabe Zimmerman, a Giffords aide killed in the 2011 shooting.

Kelly could be heard pressing Republican Sen. Jeff Flake, an fellow Arizonan lawmaker who drove from Phoenix to Tuscon to be at Giffords’s side beside after she was shot. Flake has said he will oppose the Manchin-Toomey proposal.

“Without a doubt, this bill saves lives,” Kelly said to Flake, a close friend.

The vast majority of GOP senators, along with the National Rifle Association, oppose the Manchin-Toomey proposal, and there is little indication the Republican-controlled House would take up the bill even if it passes the Senate.

In recent days, Biden has been calling red-state Democrats like Sen. Mary Landrieu of Louisiana to press her to back the Manchin-Toomey proposal. Obama was also expected to weigh in with similar personal appeals to Democrats in the hours leading up to Wednesday’s vote.

“I’m still considering it. … I haven’t made a final decision,” Landrieu said. “I have talked to Vice President Biden. We talked generally about the issue, and [I] was happy to talk with him.

Biden told reporters he was optimistic.

“We are working to get to 60 and it’s fluid,” Biden said. “I think we’re there, but it’s not unusual as you all know for people to make up their mind at the last minute, so we’ll see.”

Reid described a very emotional closed-door gathering of Senate Democrats with Giffords and Kelly. Connecticut Democrats Richard Blumenthal and Chris Murphy, who have been pressing for new gun control measures since the following the tragic Sandy Hook Elementary School shootings in December that killed left 20 childrendead, pleaded for support.

Sen. Tim Kaine (D-Va.), who was governor of Virginia when a gunman killed 32 people at Virginia Tech in 2007, also made an emotional plea.

Sen. Dianne Feinstein (D-Calif.), who has sought to ban some types of assault weapons, also implored her colleagues to support the bipartisan agreement.

“It may not be everything I want, but it’s a step in the right direction,” Feinstein said, according to Democratic sources

At this point, only four Senate Republicans — Toomey, Collins, and Sens. Mark Kirk of Illiniois (Ill.) and John McCain of Arizona (Ariz.) — are backing the Manchin-Toomey plan, or have signaled they may may do so.

Sen. Kelly Ayotte of New Hampshire is considered a swing vote because she was among the 16 Republicans who last week crossed the aisle and voted to begin debate on the gun bill.

Another one of those 16, Sen. Dean Heller (R-Nev.), said late Tuesday he will oppose the measure. Heller’s opposition could prove decisive, as McConnell has a solid 40 votes against Manchin-Toomey, according to public whip counts. One more vote would put McConnell over the top.

“Despite the good faith efforts of Senators Manchin and Toomey, the onerous paperwork and expansion of federal power mandated in this legislation are too great of a concern,” Heller said in a statement. “I believe that this legislation could lead to the creation of a national gun registry and puts additional burdens on law-abiding citizens.”

Several Democrats have so far not said whether they will vote for the proposal either, including Landrieu, Begich and Sens. Mark Pryor of Arkansas and Max Baucus of Montana. All four are up for reelection in 2014 in heavily red states.

Newly elected Democrats Heidi Heitkamp of North Dakota and Joe Donnelly of Indiana are also on the fence.

Sen. Frank Lautenberg (D-N.J.), 89, has told Reid that he will make it back to Capitol Hill for Wednesday’s action. Lautenberg has been absent from the Senate for weeks due to serious health concerns.

Ginger Gibson contributed to this report.