Less than 24 hours earlier Brown was asked about a possible campaign in New Hampshire. Shaheen is up for reelection in 2014.
"I don't think I'm done with politics, but I'm not going to rule out anything right now because I really haven't thought a heck of a lot about it," Brown said, according to CNN.
In her fundraising email, Gillibrand described Brown, who lost reelection to now-Sen. Elizabeth Warren (D-Mass.) in 2012, as a "billionaire-backed Republican."
"You read that right: A billionaire-backed Republican from Massachusetts who voted against equal pay and critical job-creation bills wants to take on Jeanne Shaheen – a tremendous champion for our progressive values," Gillibrand writes in the email.
"Brown raised $37 million in 2012 and spent it on partisan negative attacks against Elizabeth Warren."
Gillibrand's goal in the email is to raise a total of $100,000 to "send a message to Scott Brown and his Tea Party allies that we have [Shaheen's] back."
Sen. Gillibrand fundraises off of potential Scott Brown run in NH
"If he enters this race, Jeanne will immediately become the top target of Karl Rove and the Tea Party Super PACs," Gillibrand wrote in the fundraising email sent out on Friday. "Jeanne needs our help now to prepare for the attacks from Brown and his allies. They’ve launched a 'Granitebomb' campaign to fight back."