Hernandez tweeted a photo of him handing a letter to Flake.
Giffords and her husband, Mark Kelly, were at odds with Flake on tackling gun violence earlier this year.
In March, Giffords’s pro-gun control group, Americans for Responsible Solutions, ran ads in Arizona and Iowa pressuring Flake and Sen. Chuck Grassley (R-Iowa) to support universal background checks for all firearm purchasers.
“We have a problem — where we shop, where we pray, where our children go to school," Giffords said in the ad, while statistics showing high support for background checks flashed on the screen. "But there are solutions we can agree on, even gun owners like us. Take it from me — Congress must act. Let's get this done."
In April, Flake voted to allow the Senate to debate gun control legislation. A few days later, Kelly said he would support an opponent to Flake if the “right candidate” were available and if Flake didn’t support a background check bill.
Sens. Joe Manchin (D-W.Va.) and Pat Toomey’s (D-Pa.) universal background checks proposal died in the Senate that month. The final vote was 54-46.
Flake voted against it.
A poll taken after the vote by Public Policy Polling survey said his approval rating among Arizona voters was 32 percent.
--This report was updated at 7:12 p.m.