WASHINGTON — Speaker Nancy Pelosi does not believe President Trump can be removed through impeachment — the only way to do it, she said this week, is to defeat him in 2020 by a margin so “big” he cannot challenge the legitimacy of a Democratic victory.

That is something she worries about.

“We have to inoculate against that, we have to be prepared for that,” Ms. Pelosi said during an interview at the Capitol on Wednesday as she discussed her concern that Mr. Trump would not give up power voluntarily if he lost re-election by a slim margin next year.

Sitting in her office with its panoramic view of the National Mall, Ms. Pelosi — the de facto head of the Democratic Party until a presidential nominee is selected in 2020 — offered Democrats her “coldblooded” plan for decisively ridding themselves of Mr. Trump: Do not get dragged into a protracted impeachment bid that will ultimately get crushed in the Republican-controlled Senate, and do not risk alienating the moderate voters who flocked to the party in 2018 by drifting too far to the left.

“Own the center left, own the mainstream,” Ms. Pelosi, 79, said.

“Our passions were for health care, bigger paychecks, cleaner government — a simple message,” Ms. Pelosi said of the 40-seat Democratic pickup last year that resulted in her second ascent to the speakership. “We did not engage in some of the other exuberances that exist in our party” — a reference to some of the most ambitious plans advocated by the left wing of her party and some 2020 candidates, including “Medicare for all” and the Green New Deal, which she has declined to support.