Democrats are targeting vulnerable Senate Republicans over the issue of gun control in light of last week's mass shooting at an Orlando gay nightclub.

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In their efforts to regain the majority of the upper chamber,Democratic Senate candidates are highlighting GOP incumbents' opposition to a measure that would bar suspected terrorists from purchasing firearms as well as one to expand background checks.

Their renewed push sets the stage for a major clash with Republicans on Monday when votes on several gun control measures are expected.

The Senate is scheduled tovote on four proposals -- two from Democrats and two from Republicans -- after Democrats staged a nearly 15-hour filibuster to force a debate on this issue.

Democrats are rallying behind Sen. Dianne Feinstein Dianne Emiel FeinsteinMcConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Intensifying natural disasters do little to move needle on climate efforts MORE’s (D-Calif.) proposal that would permit the U.S. attorney general to block the sale of a gun if there is a "reasonable suspicion" that someone has been or will be involved in a terrorist attack.

Republicans are expected to back Sen. John Cornyn John CornynCalls grow for Biden to expand election map in final sprint Bipartisan praise pours in after Ginsburg's death Chamber of Commerce endorses McSally for reelection MORE’s (R-Texas) alternative, which would allow the attorney general to delay suspected terrorists from buying a gun for up to 72 hours while seeking a court order to stop the sale.

There will also be votes on competing background check proposals from Sen. Chuck Grassley Charles (Chuck) Ernest GrassleySenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE (R-Iowa) and Democratic Sens. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (Conn.), who started the filibuster, Chuck Schumer Chuck SchumerVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Graham signals support for confirming a Supreme Court nominee this year Pelosi orders Capitol flags at half-staff to honor Ginsburg MORE (N.Y.) and Cory Booker (N.J.).

But the arguments over the bills have already spilled into the vulnerable Senate races.

The first race to see the issue highlighted was the heated Pennsylvania Senate race between Sen. Pat Toomey (R-Pa.) and Democratic challenger Katie McGinty.

At a press conference the day before the filibuster, McGinty urged Toomey to support Feinstein’s legislation and said he “failed the test of leadership” on gun safety.”

Although Toomey hit back at her accusations noting that he had spearheaded a 2013 bill withSen. Joe Manchin Joseph (Joe) ManchinMomentum growing among Republicans for Supreme Court vote before Election Day Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' Barrett seen as a front-runner for Trump Supreme Court pick MORE (D-W.Va.) to strengthen background checksand faulting her own stances, hecame to the Senatefloor the day of the filibuster to call on his colleagues to compromise on the issue.

While knocking Feinstein’s proposal as “badly flawed,” he said Cornyn’s measure -- which he had previously voted for -- likely didn’t give enough leeway to the attorney general.

"There's an obvious opportunity here, guys, to work together and find the solution," he said. "What I'm suggesting is let's get to work here."

On Thursday, Toomey proposed his own compromise bill aimed at finding common ground between the competing measures, but his legislation wasn’t scheduled for a vote on Monday, according to the Philadelphia Inquirer. The Inquirer also reported Friday that Toomey will not vote for a Democratic-backed background check bill.

McGinty’s campaign spokesman Sean Coit in a press release on Friday accused the GOP senator of caring more about reelection over limiting gun violence.

“Not only will Toomey vote once again against legislation to close the terror loophole, but he will also vote against a bill to expand background checks on gun purchases,” Coit said. “Make no mistake about it: Toomey is all about saving his own re-election campaign, not protecting communities from gun violence."

Democrats are also taking aim at OhioSen. Rob Portman Robert (Rob) Jones PortmanRomney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery House passes B bill to boost Postal Service MORE after his muddied response last week about where he stands on legislation to ban suspected terrorists being able to buy guns.

In 2015, Portman supported Cornyn’s bill and opposed Feinstein’s.

But in a Tuesday call with reporters, Portman, facing a tough reelection campaign against former Ohio Gov. Ted Strickland (D), seemed to indicate that he was open to a bill that would ban these purchases.

When reporters asked if he was reversing where he stood last year, he said he wasn’t and explained that he opposed Feinstein’s measure because it only included those on the no-fly list.

After reporters explained that the California senator’s bill was broader, Portman sought to clarify his understanding of the 2015 bill.

“I stand corrected if you guys are right about that,” he said on the call. “I was told it was the no-fly list and it did not require” any investigation if a person claimed he or she shouldn't have been on the list, according to the Cincinnati Enquirer.

Portman aides told the New York Times Wednesday that his position remains “unchanged” and he will continue to back the Cornyn measure.

Democrats seized on Portman’s clouded explanation and called it a “24-hour flip flop” following his press call.

“It’s all just more proof that Portman is the ultimate Washington insider who will say anything to try and help himself -- but Portman’s hypocrisy and D.C. double-talk represents exactly what Ohioans hate about the dysfunctional politics of Washington,” Ohio Democratic Party spokesman Daniel van Hoogstraten said in a Wednesday statement.

Two other vulnerable Republicans are working to deflate accusations by their Democratic challengers.

Sen. 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 (R-Ill.), who’s considered the most vulnerable incumbent this cycle, was the only Senate Republican in late 2015to back Feinstein’s proposal. On Thursday, he introduced his own bill with Sen. Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE (D-Fla.) that would alert the FBI if a suspected terrorist attempts to purchase a firearm.

“Terrorists should not be able to buy weapons and the FBI should be notified if a suspected terrorist buys a weapon,” Kirk said in a statement. "This commonsense legislation equips the FBI with a new tool to stop threats on American lives.”

Still, his Democratic opponent Rep. Tammy Duckworth has hammeredKirk and chided him in a video for not participating in Democrats’ filibuster on Wednesday, saying “silence is unacceptable.”

“Tell Mark Kirk to speak on the Senate floor about preventing gun violence and to support the gun legislation coming up for a vote,” Duckworth said in the video released Thursday. "For Mark Kirk to continue representing our state while failing to show leadership on this issue is shameful."

But on Friday, Kirk announced in a statement that he was joining Feinstein as an author of her amendment. “If you are too dangerous to fly on a civil aircraft, you should not be able to buy a gun,” Kirk said. “We should learn the lessons from Orlando and keep weapons out of the hands of those who want to kill Americans.”

The other Republican isNew Hampshire Sen. Kelly Ayotte Kelly Ann AyotteBottom line Bottom line Bottom Line MORE, whose Democratic challenger, Gov. Maggie Hassan, is pushing her to reverse her opposition to preventing suspected terrorists from buying guns and support background checks. The New Hampshire Democratic Party tweeted a petition calling on Ayotte “to close the terrorist gun loophole.”

Ayotte, along with a two other GOP senators, is working with Sen. Susan Collins Susan Margaret CollinsJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-Maine) on compromise legislation that would, among its provisions, prohibit the sale of guns to terror suspects whose names appear on either the federal no-fly list or the so-called “selectee” list.

While gun control is currently a hot topic and remains high in the headlines, political observers noted that it tends to not be a high priority issue for voters when they head to the polls.

“it’s an issue that voters have a sense about, but here’s what the problem has been in the past -- it’s never ranked high in terms of vote choice,” said Terry Madonna, a public affairs professor and polling director at Franklin & Marshall College.