Lewis, a civil rights icon, has been the representative for Georgia's Fifth Congressional District since 1987.
"We are all praying for you following this diagnosis," House Speaker Nancy Pelosi posted on Twitter. "John, know that generations of Americans have you in their thoughts & prayers as you face this fight. We are all praying that you are comfortable. We know that you will be well."
Former President Obama also tweeted a message of encouragement.
"If there's one thing I love about @RepJohnLewis, it's his incomparable will to fight. I know he's got a lot more of that left in him. Praying for you, my friend," Mr. Obama wrote.
According to the latest data from the American Cancer Society, the survival rate over five years for all forms pancreatic cancer at 9%.
Pancreatic cancer is the 10th most commonly diagnosed cancer in men in the United States, and the ninth most common in women. It accounts for about 3% of all cancers but about 7% of all cancer death. It is one of the deadliest forms cancer because there are no screening tests that catch it in its early stages.
As with other cancers, pancreatic cancer is categorized by stages which describe how far the cancer has spread and how much of the body has been affected.is the most severe, meaning the cancer has spread to other parts of the body such as the liver, lungs or bones.