Republican Sens. Jeff FlakeJeffrey (Jeff) Lane FlakePoll: McSally holds 14-point lead in Arizona GOP Senate primary GOP senators introduce bill to prevent family separations at border McCain, Coons: Trump should withdraw controversial refugee nominee 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 Finance: Senators introduce bill to curb Trump's tariff authority | McConnell calls it 'exercise in futility' | Kudlow warns WTO won't dictate policy | Mulvaney feud with consumer advocates deepens Senators introduce bill to curb Trump's tariff authority Trump VA pick boosts hopes for reform MORE (Ga.), Dan CoatsDaniel (Dan) Ray CoatsGOP senator places hold on Trump counterintelligence nominee Civil liberties groups press Trump administration on NSA call record collection Trump’s ‘Syraqistan’ strategy is a success — and a failure MORE (Ind.), Chuck GrassleyCharles (Chuck) Ernest GrassleyOn The Money — Sponsored by Prudential — Markets roiled by Trump's new tariff threat | Trump lashes out at Canada over trade | Warren looks to block Trump pick for consumer agency The Hill's Morning Report — Sponsored by PhRMA — Defiant Trump meets with House GOP amid border blowback Republican senator calls for face-to-face with EPA’s Pruitt MORE (Iowa), Susan CollinsSusan Margaret CollinsActress Marcia Gay Harden urges Congress to boost Alzheimer's funding 13 GOP senators ask administration to pause separation of immigrant families Trump plan to claw back billion in spending in peril MORE (Maine), Dean HellerDean Arthur Heller13 GOP senators ask administration to pause separation of immigrant families The Hill's Morning Report — Sponsored by PhRMA — Defiant Trump meets with House GOP amid border blowback 20 weeks out from midterms, Dems and GOP brace for surprises MORE (Nev.), Kelly AyotteKelly Ann AyotteErnst, Fischer to square off for leadership post The 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 MORE (N.H.), Rob PortmanRobert (Rob) Jones Portman13 GOP senators ask administration to pause separation of immigrant families Lawmakers, businesses await guidance on tax law Harvard biz school honors Wilbur Ross MORE (Ohio) and Ted CruzRafael (Ted) Edward CruzConservatives express concern over House GOP immigration bill GOP senators introduce bill to prevent family separations at border Senate GOP tries to defuse Trump border crisis MORE (Texas), as well as Democratic Sen. Martin HeinrichMartin Trevor HeinrichOvernight Energy: DNC to reject fossil fuel donations | Regulators see no security risk in coal plant closures | Senate committee rejects Trump EPA, Interior budgets Energy commission sees no national security risk from coal plant closures The Hill's Morning Report — Trump: `A very great moment in the history of the world’ 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 HolderUS law is not on the side of Mueller's appointment as special counsel Holder redistricting group backs lawsuits for 3 additional majority-black congressional districts Liberal groups launches ads against prospective Trump Supreme Court nominees MORE will address the issue on Tuesday at the White House, and first lady Michelle ObamaMichelle LeVaughn Robinson ObamaHeckler yells ‘Mr. President, f--- you’ as Trump arrives at Capitol Meghan McCain calls out Ivanka Trump for silence on family separation policy The Hill's 12:30 Report — Sponsored by Delta Air Lines — Trump to meet House GOP as backlash to 'zero tolerance' policy grows 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.