WASHINGTON (CNN) -- Former Massachusetts Gov. Mitt Romney called for a massive military buildup of U.S. forces Thursday while campaigning in Chicago, the second time this week the GOP presidential candidate has said the U.S. military needs to be strengthened.

At a press conference in the Windy City, Romney said that the U.S. military is understaffed and called for adding "some 100,000 personnel to our troops."

He also said the military is "under-invested" and its budget should be raised to nearly four percent of the nation's gross domestic product.

"I think America has always shown that we could lead the world, but we need to have a strong military to do so," Romney said. "I think we have under-invested in our military -- not only additional troops but in my view additional purchases for armament and protective equipment and strategic defense."

Romney made similar comments on Tuesday, when he spoke at the George Herbert Walker Bush Presidential Library in College Station, Texas.