Former House Speaker Newt Gingrich compared Donald Trump to a rookie quarterback in the biggest game of his life and said it’s time for the billionaire to up his game if he has any hope of winning the White House in 2016.

Gingrich, who previously said Trump did a “very healthy” thing by expressing regret over his Heidi Cruz retweet, said the front-runner needs to get ready to be challenged on his positions as the race continues to progress.

“He’s done a lot of things right, but he also is like a rookie quarterback in the Super Bowl,” Gingrich said.

“This is the big leagues. It gets much tougher as it goes on. But he is going to have to improve his game if he’s gonna win in the end.”

Whether or not Trump can do it is largely in question.

“I think he’s a very smart man, but I also think, you know, when you’ve been as successful as he has in business, in television, in books, it’s a little hard to change at this stage because all of those other things have worked for so long,” Gingrich said of Trump’s brash personality.

Trump has often been criticized for being light on details of how he’d carry out some of his plans as president, but on Tuesday, Trump’s campaign said he’ll be giving a series of policy speeches in the coming weeks.

According to The Washington Post, topics will include how Trump plans to strengthen the military and education reforms in what campaign manager Corey Lewandowski said is a “natural maturation of the campaign.”

Gingrich said it’s something Trump should have done weeks ago, pointing to his American Israel Public Affairs Committee speech on March 21, where he changed course and used a teleprompter.

“Lots of substance,” Gingrich said of the AIPAC speech. “If he gave a series of six or eight or 10 speeches over the next month that outline what a Trump administration would be like,” Gingrich said, “I think it would make people more reassured, and I think it would give them a better sense of what the serious underlying policy components are of what has up to now been an amazing populist campaign.”