Chicago Mayor Rahm Emanuel on Thursday warned off fellow Democrats from nominating Rep. Beto O'Rourke for president in 2020, because he was a "loser" in his Texas Senate race this year.

“Nancy Pelosi led the Democratic Party for the last two years from a really bad election in 2016. I'm from Chicago, maybe I'm really old school, but to the victor go the spoils," said Emanuel, a former Obama White House chief of staff and Illinois congressman who helmed the Democratic campaign committee the last time the party flipped the House, in 2006.

O'Rourke, a three-term House member, lost a high-profile Senate bid against Republican incumbent Ted Cruz, though by a narrower margin than most Democrats in Texas seeking statewide office. O'Rourke has left the door open to a 2020 White House bid.

[Opinion: Beto O'Rourke would have a chance against Trump in 2020]

"If Beto O'Rourke wants to go and run for president, God bless him, he should put his hat in and make his case. But, he lost. You don’t usually promote a loser to the top of party and then take a winner and say, 'We’re going to take your knees off.'”

Emanuel made the comments on MSNBC Thursday as he offered his endorsement of Pelosi as the presumptive next House speaker. The California Democrat was a key ally of the Obama administration when Emanuel worked on the then-president's behalf to push his signature healthcare law bill through Congress.

"Everybody in our party always says the Republicans are so much tougher, so much stronger," Emanuel said. "Do you think they would take somebody that won and say, 'You know what your reward is? We're going to cut your knees off."'

Emanuel, known for his aggressive approach to politicking, said, "That's not how you play politics to win," especially ahead of the 2020 presidential contest. Instead, he suggested "using the foothold" Democrats have in Congress since winning the House in the 2018 midterm elections to "drive a wedge" between President Trump and Senate Majority Leader Mitch McConnell, R-Ky.

Emanuel announced in September that he would not seek a third four-year term in office ahead of the scheduled Feb. 26, 2019 election.