They'll fight it out on the presidential debate stage next week, but Bill de Blasio and Senator Kirsten Gillibrand are sparring early.

Days after he was kept out of an event to mark the passage of landmark legislation to authorize the 9/11 Victims Compensation Fund, de Blasio said he found it “unbelievable.”

"You have to ask the Senator. It makes no sense to me," de Blasio said when asked why he was not at the event.

According to the mayor's aides, de Blasio was told he was not welcome at an event led by Senator Kirsten Gillibrand, the primary sponsor of the bill. De Blasio was already in DC that day for presidential campaigning, when he tried to join the event, he was denied.

"My team made very clear that we wanted to go there to honor our first responders and to show appreciation to members of the Congress for getting this done," de Blasio said.

De Blasio and Gillibrand, both in a crowded field of Democrats running for the White House, are struggling to keep their campaigns alive.

A recent Quinnipiac university poll asked 500 Democratic voters in the swing state of Ohio who they would vote for if the Democratic primary was being held today. None listed de Blasio or Gillibrand.

Next week's debate could offer de Blasio and Gillibrand some much needed life to their struggling campaigns.

Jon Reinish, a political consultant, said it's unlikely de Blasio will target Gillibrand on the stage, but the presidential competition is always hanging in the air.

"I think there is other competition for them, but you never know. Even though this is government business for him as mayor and for her as Senator, the presidential is always playing out in the background," Reinish said.

Aides to Gillibrand denied she was trying to snub de Blasio. The event, they said was meant to focus on the first responders and their families who for years have fought to get the bill passed. According to Gillibrand's office, the mayor's staff didn’t reach out until the morning of the event when a list of speakers had already been finalized.

Glen Caplin, a senior advisor for Gillibrand, said “it's surprising that the mayor still doesn't understand that this event was about the heroes who have fought for 15 years to get this done, not a last minute photo opportunity."

De Blasio and Gillibrand will debate in Detroit on Wednesday, but they will share the stage with other big names including Joe Biden and Kamala Harris.