Democratic presidential hopefuls Sen. Kamala Harris of California and former Texas Rep. Beto O'Rourke stand the best chance of giving President Donald Trump a tough fight during the 2020 election, former White House chief strategist Steve Bannon told CNBC on Friday.

Bannon, who said he doesn't expect Trump to lose in 2020, said he would put the two Democrats on a combined ticket with Harris running for president and O'Rourke for vice president. They would be "a way to mobilize their base and give it their best shot," he told "Squawk Box."

Harris is considered a front-runner in a crowded primary field. Various early 2020 Democratic primary polls have showed Harris along with Sen. Elizabeth Warren, Sen. Bernie Sanders and former Vice President Joe Biden near the front of the pack. Though, Biden has not yet declared whether he will launch a campaign.

Harris, a self-described "progressive prosecutor" who served as attorney general of California, said her chief goal will be boosting the income of working-class people, while O'Rourke, a rising star in the Democratic Party and considered a moderate, has several key priorities, including making improvements to the Affordable Care Act.

Bannon also speculated that if Democrats don't have a presidential contender that has "broken out of the pack" by the end of this year, the Democratic Party will begin to look outside the current pool. That includes asking Hillary Clinton to run, again, the strategist contended.

"People should not count her out," Bannon said. "She's going to be sitting in the bullpen waiting for the call."

Bannon was fired as Trump's top advisor in August 2017 after serving for less than a year. His relationship with the president later took a bad turn in early 2018, when author Michael Wolff published his tell-all book "Fire and Fury."

During his tenure at the White House, Bannon was in charge of advising the president on how to best implement his nationalist agenda that appealed to the base of the Republican Party during the 2016 presidential election.

— CNBC's Jacob Pramuk and Brian Schwartz contributed to this report.