Zuckerman said personal and political reasons motivated his decision not to run.

"I was encouraged by many major political figures in New York to look closely at running for Senate," Zuckerman said. "However, at this time, it is very difficult to see how I can devote the necessary time to either a campaign, or to working in Washington, if I were to win."

Zuckerman was mentioned as a potential opponent of Sen. Kirsten GillibrandKirsten GillibrandOPINION | Hey Dems, Russia won't define 2018, so why not fix your party's problems instead? Dems see huge field emerging to take on Trump LGBT group defends general after transgender comments MORE (D-N.Y.) in this fall's midterms. The billionaire real estate mogul and owner of the U.S. News did not rule out a run two weekends ago amidst rumors he would run as an independent or Republican candidate.

It's the second time this week Gillibrand has lost a potential, high- profile challenger. Former Rep. Harold Ford, Jr. (D-Tenn.) announced this week he would not primary the appointed senator. 

Democrats acknowledged Gillibrand's positives just after the Zuckerman announcement was made public. Gillibrand has suffered from low polling numbers and name recognition, which invited potential challengers.

"In just a short time, Senator Gillibrand  has proven herself to be an effective fighter for New York," Democratic Senatorial Campaign Committee Chairman Robert MenendezRobert MenendezBipartisan group, Netflix actress back bill for American Latino Museum The Mideast-focused Senate letter we need to see Taiwan deserves to participate in United Nations MORE (D-N.J.) in a statement. "She's working hard, getting around the state, building coalitions of support - including some who were initially skeptical upon her appointment. But as we have seen time and time again, she is New York tough and she is impressing New Yorkers every chance she gets."