If President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE endorses gun-control legislation, it will take 13 Senate Republican votes to pass the measure, assuming the entire 47-member caucus of Democrats and Independents backs it.

Conservatives such as Sen. Ted Cruz Rafael (Ted) Edward CruzCrenshaw looms large as Democrats look to flip Texas House seat SCOTUS confirmation in the last month of a close election? Ugly The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' MORE (R-Texas) say any proposal that goes as far as the 2013 amendment sponsored by Sens. Joe Manchin Joseph (Joe) ManchinThe debate over the filibuster entirely misses the point Trump plans to pick Amy Coney Barrett to replace Ginsburg on court Day before Trump refused to commit to peaceful transition, Aaron Sorkin described how he would write election night MORE (D-W.Va.) and Pat Toomey Patrick (Pat) Joseph ToomeyAppeals court rules NSA's bulk phone data collection illegal Dunford withdraws from consideration to chair coronavirus oversight panel GOP senators push for quick, partial reopening of economy MORE (R-Pa.) will face a backlash from the right, meaning 60 votes will be necessary to break a filibuster.

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The Manchin-Toomey proposal, which is basis of current Senate negotiations, would expand back to include all sales over the Internet and at gun shows but exempt sales between family, friends and coworkers.

Here are the 13 Republicans crucial to passing gun-control legislation:

Sen. Pat Toomey (R-Pa.)

Toomey is the lead Republican sponsor of Manchin-Toomey, which was first offered after the 2012 mass shooting at Sandy Hook Elementary School in Newtown, Connecticut.

His amendment was negotiated in 2013 in an attempt to win support from the National Rifle Association, which ended up opposing it.

Toomey was one of four Republicans to vote for the measure, along with Sen. Susan Collins Susan Margaret CollinsDemocratic senator to party: 'A little message discipline wouldn't kill us' Poll: 57 percent of Americans think next president, Senate should fill Ginsburg vacancy On The Trail: Making sense of this week's polling tsunami MORE (Maine) and Sens. John McCain John Sidney McCainCrenshaw looms large as Democrats look to flip Texas House seat Analysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture MORE (Ariz.) and Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE (Ill.). McCain and Kirk are no longer in the Senate.

Toomey has spoken to Trump several times since the mass shootings in Dayton, Ohio, and El Paso, Texas, last month gave new urgency to the issue of gun violence.

“It’s time to act. This is the time to do what we can to help make our communities as safe as we can. There is a bipartisan opportunity to expand background checks to cover all commercial sales. And while there have been many ideas that been floated, this is one that I think has the most resonance, the most saliency and the best chance of becoming law,” Toomey told reporters in his office last week.

Sen. Lindsey Graham Lindsey Olin GrahamLincoln Project mocks Lindsey Graham's fundraising lag with Sarah McLachlan-themed video The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-S.C.)

Graham is chairman of the Judiciary Committee, which has primary jurisdiction over gun control, and is a close ally of Trump.

He voted against Manchin-Toomey in 2013 and 2015 and expressed some ambivalence earlier this past week when asked about expanding background checks.

Graham, however, also told reporters that Trump is interested in expanding background checks and will likely endorse whatever proposal the president puts forth.

He is weighing legislation sponsored by Toomey and Sen. Chris Coons Christopher (Chris) Andrew CoonsTrump's push for win with Sudan amps up pressure on Congress Murkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight MORE (D-Conn.) to require that law enforcement officials are notified if someone who attempts to purchase a firearm fails a background check.

He also is working with Sen. Richard Blumenthal (D-Conn.) on so-called “red flag” legislation that would provide grants to states to empower law enforcement to confiscate firearms from individuals judged to be dangerous to themselves or others.

Graham faces re-election next year and will be cautious about endorsing any proposal that significantly expands background checks, an idea likely to spark controversy in conservative South Carolina.

Sen. Susan Collins (R-Maine)

Collins in one of only two Republicans currently in the Senate who has backed Manchin-Toomey.

Even if Trump doesn’t back a gun-control measure, Collins might support it.

She faces re-election next year in a state won by Hillary Clinton Hillary Diane Rodham ClintonBloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE in 2016 and has come under fire from Democrats for confirming Justice Brett Kavanaugh Brett Michael KavanaughTrump plans to pick Amy Coney Barrett to replace Ginsburg on court Collins trails challenger by 4 points in Maine Senate race: poll SCOTUS confirmation in the last month of a close election? Ugly MORE to the Supreme Court.

Sen. Mitt Romney Willard (Mitt) Mitt RomneyCrenshaw looms large as Democrats look to flip Texas House seat The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-Utah)

Romney told reporters last week that all commercial gun sales should be subject to background checks and expressed willingness to support legislation along the lines of Manchin-Toomey.

“It certainly should be applied to commercial sales and finding a more comprehensive way to make sure that people are in the system that ought to be in the system,” Romney told reporters Monday.

Romney told The Hill on Thursday that he is talking with Manchin and Toomey about possible changes to their legislation to make it easier for people in rural areas to comply with background-check requirements when they buy or sell firearms from individuals who are not licensed gun dealers.

“They’re still working on some of the very detailed elements of it and I have spoken with them in some depth. I’m not looking for changes, I’m looking to see where they land. One of the areas of concern of course is how to make sure any kind of background-check technology would contemplate people in rural areas that live a long way from a licensed dealer that might have access to a background check terminal,” Romney said.

Sen. Mitch McConnell Addison (Mitch) Mitchell McConnellDemocratic senator to party: 'A little message discipline wouldn't kill us' House to vote on resolution affirming peaceful transition of power Republican lawyers brush off Trump's election comments MORE (R-Ky.)

Multiple Republican senators say McConnell is involved in the talks to put together a gun-violence package and has signaled interest in passing legislation as long as it has Trump’s support.

“He would like to get something done,” said a Republican senator involved in the talks.

McConnell told WHAS radio in Kentucky last month: “What we can’t do is fail to pass something.”

“The urgency of this is not lost on any of us,” he said.

A GOP poll last month showed that women voters in key suburban districts rate working to prevent gun violence as their top issue and that 72 percent of suburban women say gun laws should be more strict. McConnell in an interview with reporters in April identified suburban voters as the key to keeping Republican control of the Senate in 2020.

McConnell last month asked three chairmen, Graham, and Senate Health Committee Chairman Lamar Alexander Andrew (Lamar) Lamar AlexanderPelosi urges early voting to counter GOP's high court gambit: 'There has to be a price to pay' Graham: GOP has votes to confirm Trump's Supreme Court nominee before the election The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Tenn.) and Commerce Committee Chairman Roger Wicker Roger Frederick WickerThe Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Key Democrat opposes GOP Section 230 subpoena for Facebook, Twitter, Google MORE (R-Miss.) to explore various legislative proposals to respond to recent mass shootings.

Publicly, McConnell has kept his cards close to the vest, declining to tell reporters what policies he would support before Trump weighs in with his own recommendations.

“I’m going to wait and assess the proposal that actually could become law and at that point I’ll be happy to explain my vote one way or the other,” McConnell told reporters Tuesday.

Sen. Cory Gardner Cory Scott GardnerBreaking the Chinese space addiction Trump dumbfounds GOP with latest unforced error Billionaire who donated to Trump in 2016 donates to Biden MORE (R-Colo.)

Gardner, like Collins, faces a difficult reelection race in a state that voted for Hillary Clinton in 2016.

He wasn’t in the Senate when the chamber first voted on Manchin-Toomey in 2013 but he voted against it in a lower-profile vote in December of 2015, which Democrats forced after a mass shooting in San Bernardino, California.

Gardner this past week declined to say whether he would consider voting for Manchin-Toomey and expressed interest in knowing what changes might be made to the legislation.

“Are they talking about changing it? I haven’t looked at the recent changes,” he said.

Gun-control has had a mixed record in Colorado. Two Democratic state senators were removed from their jobs in recall elections in 2013 after the Democratic-controlled statehouse passed several gun control measures.

Sen. Martha McSally Martha Elizabeth McSallyOn The Trail: Making sense of this week's polling tsunami The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ariz.)

McSally is another top Democratic target up for re-election in 2020.

Her expected Democratic opponent is Mark Kelly, the former astronaut and prominent gun-control advocate whose wife, former Rep. Gabby Giffords (D-Ariz.) was seriously injured in a mass shooting in 2011.

Kelly, the co-founder along with Giffords of Americans for Responsible Solutions, has been at the forefront of the national debate over gun violence and is expected to make it a major issue in next year’s campaign.

McSally could insulate herself from political attacks if she supports legislation to expand background checks, which has garnered strong support in public polls, especially among suburban women.

McSally this past week declined to comment on whether she would support a variation of Manchin-Toomey or any other gun-control legislation.

“Sorry, I got to go vote,” she said as she raced past reporters to the Senate floor last week.

Sen. Lamar Alexander (R-Tenn.)

Alexander is a moderate Republican and close ally of McConnell.

He is retiring at the end of next year and would be free to vote for legislation to expand background checks or address gun violence in other ways without fear of reprisal from the National Rifle Association.

McConnell has tasked Alexander, the chairman of the Senate Health, Education, Labor and Pensions Committee, with exploring strategies under his jurisdiction to combat gun violence.

GOP senators say Alexander will focus on legislation to address mental health issues and their contribution to gun violence. Any chance of passing such a bill will rely on attaching it to some expansion of background checks that can win Democratic support.

“Our nation cannot ignore these mass shootings,” Alexander declared in a press release last month. “I am ready to do more, especially on background checks, to identify those who shouldn’t have guns.”

Sen. Thom Tillis Thomas (Thom) Roland TillisOvernight Energy: Trump officials finalize plan to open up protected areas of Tongass to logging | Feds say offshore testing for oil can proceed despite drilling moratorium | Dems question EPA's postponement of inequality training On The Trail: Making sense of this week's polling tsunami Feds say offshore testing for oil can proceed despite drilling moratorium MORE (R-N.C.)

Tillis faces a competitive election next year in which suburban voters will be crucial to victory.

Tillis hasn’t ruled out voting for a variation of Manchin-Toomey but, like McConnell, says he will wait for a recommendation from Trump before announcing his position.

Tillis says whatever legislation comes to the floor must have “proper due process for law-abiding citizens.

“Hopefully we can find a way to work to the middle and not necessarily pursue a path that just gets us into a political debate with no outcome,” he recently told reporters.

“I’ve spoken to Sen. Toomey,” he added. “I’m open to anything, again, that we can get bipartisan consensus on and also make sure we’re not overreaching and really beginning to threaten the rights of law-abiding citizens, of which the majority of people who own guns are,” he said.

Tillis said Trump’s support for gun-violence legislation, however, is “critical.”

Sen. John Cornyn John CornynHillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close Lawmakers introduce legislation to boost cybersecurity of local governments, small businesses On The Trail: Making sense of this week's polling tsunami MORE (R-Texas)

McConnell has tapped Cornyn along with the chairmen of the Judiciary, Health and Commerce Committees to explore proposals to include in a gun-violence package.

Cornyn was the lead sponsor of the last significant gun-control measure to be enacted into law, the Fix NICS Act of 2017. That bill implemented penalties for federal agencies failing to report relevant information to the National Instant Criminal Background Check System (NICS). Trump signed the bill into law last year.

Two recent mass shootings took place in Cornyn’s home state, in El Paso and Odessa, giving him a special stake in the Senate debate.

Cornyn is also up for re-election next year in a state that is trending more Democratic and where suburban voters, who tend to favor stricter gun control, will be crucial to victory.

A University of Texas/Texas Tribune poll last year showed that 54 percent of Texas voters strongly support mental and criminal background checks for all gun sales.

Asked about Manchin-Toomey, Cornyn said: “I’m not adverse to that — to considering, to voting on it but there are a lot of other things I think would be in the mix.”

Sen. Rob Portman Robert (Rob) Jones PortmanMcConnell locks down key GOP votes in Supreme Court fight Romney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery MORE (R-Ohio)

Portman represents Dayton, Ohio, where ten people were killed by a gunman on Aug. 4 and has a long track record of working on bipartisan deals.

Portman voted against Manchin-Toomey in 2013 and is keeping abreast of discussions on how to modify that proposal to pick up more GOP support.

Sen. Marco Rubio Marco Antonio RubioGOP lawmakers distance themselves from Trump comments on transfer of power McConnell pushes back on Trump: 'There will be an orderly transition' Graham vows GOP will accept election results after Trump comments MORE (R-Fla.)

Rubio’s home state was the site of one of the nation’s most high-profile mass shootings last year, when a gunman killed 17 people and injured 17 others at Marjory Stoneman High School in Parkland, Florida.

Rubio and fellow Florida senator Rick Scott (R) have come under pressure from the parents of students killed in Parkland to support gun-control legislation.

Rubio hasn’t ruled out voting for a version of Manchin-Toomey but he says legislation to empower police to confiscate firearms from individuals deemed to be dangerous would be a more effective way to prevent future tragedies.

“I just think it would be more effective. If we knew who the people were that were a threat and were able to get to them before they took action, that would be far more effective in preventing the tragedies that have brought us to this point. And I also think it’s a bill that can pass,” he said.

Sen. Lisa Murkowski Lisa Ann MurkowskiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Energy: Trump officials finalize plan to open up protected areas of Tongass to logging | Feds say offshore testing for oil can proceed despite drilling moratorium | Dems question EPA's postponement of inequality training Poll: 57 percent of Americans think next president, Senate should fill Ginsburg vacancy MORE (R-Alaska)

Murkowksi along with Collins is a prominent moderate who has shown she’s not afraid to buck leadership on big votes, such as her vote against the 2017 effort to repeal ObamaCare.

She also demonstrated her independence last year by opposing the nomination of Justice Brett Kavanaugh to the Supreme Court.

Murkowski, however, is keeping her distance from the Senate negotiations on expanded background checks and waiting for the dust to settle.

“I know there have been talks going on, that’s as much as I know,” she told The Hill.

Murkowski also declined last week to comment about whether she would support a revamped version of Manchin-Toomey.

She voted against it in 2013.