Unable to move legislation through the last Congress — with a Senate Democratic majority — or this one, gun-control advocates in the upper-chamber have a two-pronged strategy to get stricter regulations.

First, Sens. Richard Blumenthal (D-Conn.), Chris Murphy (D-Conn.) and 11 of their colleagues are asking gun dealers to refuse to sell a firearm to anyone who does not pass a background check on the first try.

That initial background check approves a purchase, denies a purchase, or flags a person who may have a criminal record and warrants further investigation. Then the FBI steps in to review the purchaser in several databases and give a final OK to a gun sale. If the FBI takes longer than 72 hours to complete the review, the “hold” is released and the gun sale can go forward without notification from the seller to the FBI.

If the FBI issues a verdict after the three-day review period and determines the buyer shouldn’t be owning a gun, the ATF is dispatched to retrieve it.

Charleston church shooter Dylann Roof fell into the review category as a drug possession arrest was mistakenly classified as a felony, setting off the alert. A decision was never made during the review period because the wrong arresting agency was listed on his criminal record.

The Senate Dems sent a letter today to Cabela’s, EZ Pawn, and Bass Pro Shops, three retailers that allow sales after the 72-hour period if they don’t hear back from authorities, stressing that even though law gives them a “default to proceed” in these cases it doesn’t mean they have to sell to a buyer.

They argued that over the past five years 15,729 “default” firearms sales have been made to people eventually deemed ineligible to buy a gun.

“Responsible gun retailers can act today to address this unacceptable situation. The law allows retailers to decide whether or not to allow gun sales to proceed after the three-day ‘default period’ has elapsed. You have a duty to ensure that your products do not get into the hands of dangerous individuals like the Emanuel AME Church shooter,” the senators wrote. “…No responsible gun retailer should transfer a gun without first conducting a complete background check.”

On CNN today, Blumenthal said this request for voluntary withholding of gun sales needs “stronger executive action” from President Obama.

“My hope also is that the president will launch a major mental health initiative because ultimately so many of these shooters are dangerous to themselves or other and should not be in possession of a handgun or any gun because they can do such harm as we’ve seen time and again in everyone these horrific tragic shootings, like Newtown, Charleston, Lafayette,” he said.

“If that loophole were closed, and licensed gun dealers were required to wait for all of the background checks to be approved, that sale probably never would have proceeded,” the senator added of Charleston. “So the loophole does matter. On the other hand, the idea that guns should be sold to people who have histories of involuntary commitments or other kinds of very serious mental health issues, I think ought to be seriously examined.”

Blumenthal added that lawmakers should still act as it’s “not just an opportunity, an obligation because Congress becomes complicit, maybe now is complicit in the failure to prevent these very, very avoidable tragedies.”