Republican Sens. Jeff FlakeJeffrey (Jeff) Lane FlakeGOP anxiety grows over Trump’s Iran decision GOP senator: Trump's comment on Kim Jong Un 'surpasses understanding' Republican candidate favored in Arizona special House election MORE (Ariz.), Saxby ChamblissClarence (Saxby) Saxby ChamblissLobbying World Former GOP senator: Let Dems engage on healthcare bill OPINION: Left-wing politics will be the demise of the Democratic Party MORE (Ga.), Johnny IsaksonJohn (Johnny) Hardy IsaksonOvernight Defense: VA nominee on the ropes | White House signals it will fight for pick | Trump talks Syria with Macron | McConnell tees up Pompeo vote White House signals it will fight on for Trump VA pick Senator offers new details on allegations against VA nominee MORE (Ga.), Dan CoatsDaniel (Dan) Ray CoatsOvernight Defense: Lawmakers worry over Syria strategy | Trump's base critical of strikes | Flake undecided on Pompeo | Coast Guard plans to keep allowing transgender members | GOP chair wants to cut B from Pentagon agencies Pompeo faces difficult panel vote after grilling by Dems Pompeo confirms he was interviewed by Mueller MORE (Ind.), Chuck GrassleyCharles (Chuck) Ernest GrassleyLet Robert Mueller do his job Overnight Cybersecurity: Senators eye path forward on election security bill | Facebook isn't winning over privacy advocates | New hacks target health care Juan Williams: GOP support for Trump begins to crack MORE (Iowa), Susan CollinsSusan Margaret CollinsWinter Olympians call for action on climate Trump’s CIA pick facing brutal confirmation fight Senate panel to examine Trump officials' election security efforts MORE (Maine), Dean HellerDean Arthur HellerSenate GOP wary of new tax cut sequel GOP Senate hopefuls race to catch up with Dems Family, friends mourn death of Barbara Bush MORE (Nev.), Kelly AyotteKelly Ann AyotteThe Hill's Morning Report: Koch Network re-evaluating midterm strategy amid frustrations with GOP Audit finds US Defense Department wasted hundreds of millions of taxpayer dollars US sends A-10 squadron to Afghanistan for first time in three years MORE (N.H.), Rob PortmanRobert (Rob) Jones PortmanSenators debate new business deduction, debt in tax law hearing Tax rules will be subject to more OMB review under new memo Ending sex trafficking tomorrow requires preventing child abuse today MORE (Ohio) and Ted CruzRafael (Ted) Edward CruzCambridge Analytica whistleblower briefs House Dems After Dems stood against Pompeo, Senate’s confirmation process needs a revamp Cruz's Dem challenger slams Time piece praising Trump MORE (Texas), as well as Democratic Sen. Martin HeinrichMartin Trevor HeinrichTrump’s CIA pick facing brutal confirmation fight Dem senators unveil expanded public option for health insurance Senators introduced revised version of election cyber bill 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 Himpton HolderComey's book tour is all about 'truth' — but his FBI tenure, not so much James Comey and Andrew McCabe: You read, you decide Eric Holder headed to New Hampshire for high-profile event MORE will address the issue on Tuesday at the White House, and first lady Michelle ObamaMichelle LeVaughn Robinson ObamaGeorge H.W. Bush in intensive care Michelle Obama congratulates duke and duchess of Cambridge on royal baby ‘Morning Joe’ host: Trump tweeting during Barbara Bush funeral ‘insulting’ to US 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.