Former Rep. Gabby Giffords and her husband, astronaut Mark Kelly, are not betting against Senator John McCain in his fight with an aggressive brain cancer.

The couple became close six and half years ago, as Giffords fought for her life after surviving an assassination attempt in 2011. And as Kelly has seen first-hand, a positive outlook is crucial to fighting the cancer.

"It matters a lot," Kelly says. "I’ve seen it in Gabby in the last six years, especially in the beginning.

"With a significant brain injury, it is important to battle back and not give up," Kelly said.

The former Navy pilot says he has looked up to McCain his entire life.

"He is an icon as far Navy fight pilots go," Kelly said in an interview Wednesday night.

He notes that McCain never gave up during the five and half years as a prisoner of war, refusing to be released early after brutal torture.

"He is a tough guy," Kelly said. "We are incredibly optimistic on his recovery."

On Twitter Wednesday night, Giffords told McCain to "hang in there."

"You're tough! You can beat this. Fight, fight, fight! I am proud to call you my friend," Giffords tweeted.