Senate Republicans are swinging the Iran deal like a brickbat at Democratic challengers, hoping to bolster their vulnerable majority.

This week, their campaign committee began targeting Democratic candidates who haven't made up their mind on whether to support the agreement President Obama and other world powers reached with Iran to limit Tehran's nuclear weapons program. Republicans are hoping to paint Democrats as weak on national security, and improve their 2016 prospects in the process. The GOP's holds a five-seat majority, and is defending at least that many seats in swing and blue territory.

Congress is to vote on a motion to pull the U.S. out of the Iran deal, and maintain sanctions on the regime, in September. Polling has turned against the agreement, with Americans expressing opposition to the accord and skepticism that Iran will honor it. Republican operatives view these developments as a political opening. "This allows us to point to a specific example of how Democrats are weak on national security and terrorism," one party strategist said.

The Democratic Senatorial Campaign Committee is slamming Republicans for playing politics with national security.

"Every candidate is responsibly taking the time to review the deal and come to their own conclusion and the fact that the Republicans are slamming candidates for thoroughly reviewing such an important issue shows just how irresponsibly they govern," DSCC spokeswoman Sadie Weiner said.

The administration is lobbying Congress hard to defeat the motion of disapproval.

Democrats on Capitol Hill are overwhelmingly siding with Obama on the matter, although pockets of resistance exist, particularly among party members who are more hawkish on national security. Notably, Sen. Charles Schumer of New York, expected to succeed Sen. Harry Reid of Nevada as Democratic leader in 2017, will vote against the deal, as will Sen. Bob Menendez of New Jersey, a senior member of the Foreign Relations Committee.

Republicans are hoping to sew division in Democratic ranks, or pigeonhole Democratic Senate candidates as being on the wrong side of the issue, by exploiting their indecision. In a Fox News poll taken this month, 58 percent said Congress should reject the deal; 75 percent said Iran couldn't be trusted not to cheat.

On Tuesday, Senate Republicans through their campaign committee targeted Nevada's Catherine Cortez Masto, the Democrat running to succeed Reid, who is retiring in 2016. The NRSC, formerly the National Republican Senatorial Committee, said in a press release that Cortez Masto was delaying her decision on whether to support the Iran deal on orders from Reid. Cortez Masto is set to face GOP Rep. Joe Heck in November 2016.

"Harry Reid's handpicked candidate appears to be waiting on orders from her political mentor before taking a position on the Obama administration's nuclear deal with Iran," the NRSC charged. "Reid still hasn't taken a position on the deal in order provide cover to Cortez Masto and allow her to remain silent on a critical issue of national security."

The NRSC's other targets include: New Hampshire Gov. Maggie Hassan, expected to challenge Sen. Kelly Ayotte; Rep. Tammy Duckworth, D-Ill., challenging Sen. Mark Kirk; Reps. Patrick Murphy and Alan Grayson, running for the Democratic nomination in Florida and the right to compete for their state's open Senate seat; and Katie McGinty, a Senate Democratic primary candidate in Pennsylvania, where Sen. Pat Toomey is running for re-election.

New Hampshire, Illinois and Pennsylvania tend to lean Democratic in presidential elections, although the Granite State is a battleground. Florida is a swing state that voted for Obama twice. The Republicans also are defending seats in Wisconsin and North Carolina. The Democrats are defending seats in Nevada and Colorado. It's unclear at this point if the seats in Colorado and North Carolina are going to be competitive.

This strategy of hitting Democratic Senate candidates undecided on the Iran deal is filtering down to other arms of the Republican Party and its supporters. On Tuesday, the GOP-friendly America Rising super PAC attacked Duckworth on Twitter. "@AmericaRising Sen. Menendez is the latest Dem to oppose #IranDeal. When will @RepDuckworth stop ducking Qs?"

On Wednesday, the Ohio Republican Party went after former Gov. Ted Strickland, the Democrat challenging Sen. Rob Portman. Strickland's previous employer, the D.C. think tank Center for American Progress, is backing the Iran deal, and the Ohio GOP is trying to put Strickland on the spot.

"It is time for Ted Strickland to level with Ohio voters and tell us if he supports President Obama's Iran deal," state party Chairman Matt Borges said in a statement.