"I said this to the president any number of times: 'You don't have to swing at every pitch,'" said Christie, who unsuccessfully challenged Trump for the 2016 GOP presidential nomination.
Trump's "biggest weakness as a political candidate is he swings after every pitch — the one at your head, the one in the dirt, the one 4 feet outside. It just doesn't make sense in my mind to do that," Christie said.
Christie's advice to the president came during questions on "Squawk Box" about the timing of a July 25 call in which Trump is accused of asking the president of Ukraine to investigate the family of Democratic presidential frontrunner Joe Biden. The call was one day after former special counsel Robert Mueller testified in Congress about his nearly two-year investigation into Russian interference in the 2016 election.
"What it tells you again and again is that the president, this president, is going to do whatever he feels like he wants to do without worrying about what ... anybody else is going to say about it," said Christie.
"Congress has shown themselves to be completely ineffectual at doing anything to rein that in; even when they have a majority in the House," he added, despite more and more Democrats pushing for Trump impeachment proceedings. House Speaker Nancy Pelosi scheduled an all-caucus meeting in the afternoon. Impeachment is expected to be on the agenda.
Christie's relationship with Trump over the years has been up and down, at times nasty on the 2016 campaign trail and later more congenial. Christie generally voices support for Trump's economic agenda, including the president's China trade war.
Taking on China is a "worthy fight," said Christie, adding the president has been consistent on trade over the years.
"Trump has been in a whole bunch of different spots in politics over 30 years," Christie said. "But this is the one place he has been [constant], whether it was with Japan in the '80s or with Mexico then in the '90s and now with China in the 2000s. I don't think he can afford to back off now. He's got to ride this out."
The U.S. and China have been engaged in a trade war for the past year and a half, with each side upping retaliatory import tariffs over the summer. The largest hike in duties took place earlier this month before Washington and Beijing agreed to meet for another round of high-level talks in October.
When it comes to Trump's confrontational approach with Beijing, Christie said options were limited.
"We've tried almost every other approach, and it hasn't worked," said Christie. "Quite frankly, it's the right time to take this approach and try it because our economy domestically is so strong. If we weren't as strong domestically, we couldn't afford this fight."