WASHINGTON — Opponents of President Barack Obama’s nuclear pact with Iran tried turning up the heat this week on undecided Senate Democrat Jon Tester with a TV ad saying politician like him would have “blood on their hands” if they supported the deal. But the burly Montanan came out in favor, the latest of a string of Democrats to shrug off ad campaigns from opposition groups.

The ad against Tester was pretty tough, but the real money is being spent by an offshoot of the high-powered American Israel Political Action Committee, which is running ads on nationwide TV and in the states of more than a dozen undecided Democrats, among them Sens. Joe Manchin of West Virginia, Chris Coons of Delaware and Mark Warner of Virginia.

The accord would curb Iran’s nuclear program in exchange for relief from economic sanctions, and the House and Senate are slated to vote next month on a resolution of disapproval.

“Iran had signed a treaty banning torture, but they did it anyway,” says Ahmad Batebi, an Iranian human rights activist who was tortured by the regime. “Now they’ve signed a deal promising no nuclear weapons, but they keep their nuclear facilities and ballistic missiles. What do you think they’ll be doing?”

The ad by Citizens for a Nuclear Free Iran, the AIPAC-sponsored group, is tougher than an earlier one it ran. But the ad doesn’t ask viewers to take any action, like calling their senator or congressman. And it doesn’t name names. Instead, it displays the group’s website address, where those inclined to visit will find more information and get automated assistance in calling their lawmakers.

“If part of what you’re trying to do is influence members of Congress to vote against the deal – it’s not the members of Congress who are going to see this, you’re targeting it to their constituents – so I don’t understand why there isn’t a call to action at the end of the ad. You know, ‘Call Congress, call your congressman’ to generate heat that way,” said Rodell Mollineau of Rokk Solutions, who has orchestrated ad campaigns for American Bridge 21st Century, a Democratic super PAC.

So do lawmakers feel like the ads are tightening the screws? It is August, after all, when fewer eyeballs watch TV and many people are on vacation.

“No pressure at all,” said Manchin spokesman Jonathan Kott, adding that his boss is leaning toward supporting the agreement. “I know he is aware of the ads and campaigns, but he hasn’t heard about them from constituents.”

Tester announced on Thursday that he was backing the deal. “It’s clear this deal is the only option right now to stop Iran from developing a nuclear weapon,” the senator said in a statement.

The AIPAC-allied group is also running ads in the Philadelphia media market, aimed at Sens. Bob Casey, D-Pa., Coons and Cory Booker, D-N.J.