Republican Sens. Jeff FlakeJeffrey (Jeff) Lane FlakeCorker pressed as reelection challenges mount -trillion debt puts US fiscal house on very shaky ground Senate votes down Paul's bid to revoke war authorizations MORE (Ariz.), Saxby ChamblissSaxby ChamblissFormer GOP senator: Let Dems engage on healthcare bill OPINION: Left-wing politics will be the demise of the Democratic Party GOP hopefuls crowd Georgia special race MORE (Ga.), Johnny IsaksonJohn (Johnny) Hardy IsaksonHouse sends resolution urging Trump to condemn white supremacists Senate approves resolution condemning white supremacist groups The Hill's 12:30 Report MORE (Ga.), Dan CoatsDaniel (Dan) Ray CoatsOvernight Cybersecurity: DHS bans agencies from using Kaspersky software | Panel calls Equifax CEO to testify | Facebook pulling ads from fake news Mueller investigation focusing on social media's role in 2016 election: report Intelligence director criticizes former officials for speaking out against Trump MORE (Ind.), Chuck GrassleyCharles (Chuck) Ernest GrassleyGrassley: 'Good chance' Senate panel will consider bills to protect Mueller Overnight Finance: CBO to release limited analysis of ObamaCare repeal bill | DOJ investigates Equifax stock sales | House weighs tougher rules for banks dealing with North Korea GOP state lawmakers meet to plan possible constitutional convention MORE (Iowa), Susan CollinsSusan Margaret CollinsSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Ryan: Graham-Cassidy 'best, last chance' to repeal ObamaCare Collins skeptical of new ObamaCare repeal effort MORE (Maine), Dean HellerDean Arthur HellerSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Lawmakers grapple with warrantless wiretapping program Overnight Health Care: New GOP ObamaCare repeal bill gains momentum MORE (Nev.), Kelly AyotteKelly Ann AyotteStale, misguided, divisive: minimum wage can't win elections Trump voter fraud commission sets first meeting outside DC RNC chair warns: Republicans who refused to back Trump offer 'cautionary tale' MORE (N.H.), Rob PortmanRobert (Rob) Jones PortmanWeek ahead in tech: Debate over online sex trafficking bill heats up 'Hillbilly Elegy' author won't run for Senate Brown, Portman urge Trump administration to move quickly on a steel decision MORE (Ohio) and Ted CruzRafael (Ted) Edward CruzSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Overnight Finance: CBO to release limited analysis of ObamaCare repeal bill | DOJ investigates Equifax stock sales | House weighs tougher rules for banks dealing with North Korea GOP state lawmakers meet to plan possible constitutional convention MORE (Texas), as well as Democratic Sen. Martin HeinrichMartin Trevor HeinrichLive coverage: Sanders rolls out single-payer bill Where Dems stand on Sanders's single-payer bill Trade agency should listen to Congress on solar case MORE (N.M.), are being targeted in the effort.

The ads, which will run on Facebook and on search engines, ask supporters to call the individual senators and express support for expanded gun-control measures.

"Too many Americans are dying from gun violence & Congress needs to act. Call Sen. Collins!" reads the ad running against Collins.

The Senate will begin consideration of a comprehensive gun control bill this week or next; it's still unclear whether President Obama will be able to successfully rally support for one of the most politically feasible measures — universal background checks for gun owners.

Opposition from Republicans and some red-state Democrats has resulted in other Obama priorities, including an assault-weapons ban, being stripped from the main bill.

The passage of universal background checks is largely considered the test of whether Obama's push for expanded gun control measures, which followed the Sandy Hook massacre late last year, has been a success.

The president spent Monday in Hartford, Conn., speaking in support of his effort to reduce gun violence. Vice President Biden and Attorney General Eric HolderEric H. HolderJuan Williams: Momentum builds against gerrymandering GOP worries as state Dems outperform in special elections House votes to curb asset seizures MORE will address the issue on Tuesday at the White House, and first lady Michelle ObamaMichelle LeVaughn Robinson ObamaObama to host global summit in Chicago Kelly Clarkson says new song 'Go High' inspired by Michelle Obama Michelle Obama outshines all Democratic prospects for 2020 MORE will advocate for the package at a speech in Chicago later in the week.

The OFA has been working to support his lobbying effort, issuing ads against 16 GOP lawmakers earlier this year. It hopes the online ad campaigns will pressure some of the 11 senators to support the final bill.

While many of the senators targeted in the new ad push reside in red states where any expansion of gun control measures is unlikely to be politically palatable, some — like Collins and Ayotte — are in states that lean blue and might be open to the bill.

Collins is up for reelection in 2014, and a vote for the gun control bill could give her a boost among Democrats and centrists in Maine.

Chambliss, too, represents a possible GOP vote in favor of a bill, as he's retiring in 2014.

Top aides to the president’s reelection effort relaunched his campaign arm, Obama for America, as the new OFA in January. Now a nonprofit, Organizing for Action can, by law, engage in issue advocacy but not electoral politics. 

It can also receive unlimited donations, and does not have to disclose its donors.

The group has come under fire by campaign finance reform advocates who have expressed concern that special interests can buy “access” to the president through contributions to his advocacy arm, but the group has said it will voluntarily disclose donors.

--This post was updated at 11:33 a.m. on Tuesday.