As former President Donald J. Trump and Vice President Kamala Harris face off in their debate Tuesday night, one likely area of contention will be their mutual accusations of flip-flopping — a charge that politicians have long deployed to portray their opponents as lacking principle.

It is true that both have changed some of their policy positions, as politicians often do — whether for political expediency or because their thinking has evolved with new information. But while Ms. Harris has moderated a number of progressive stances she took in the 2020 Democratic primary, Mr. Trump has reversed himself entirely, gone back and forth or avoided taking clear stands on a host of important issues.

Here is a look at some areas where they have shifted noticeably.

Trump on abortion

1999-2022

Long before he ran for president, Mr. Trump described himself as “very pro-choice.” That stance changed when he decided to seek the 2016 Republican nomination. He needed evangelical voters, so he recast himself as a staunch opponent of abortion and promised to — and did — appoint Supreme Court justices who would overturn Roe v. Wade.

That he was a newcomer to the movement was clear in his call in 2016 for women who have abortions to be punished, a position anti-abortion activists tend to reject, saying it is providers who should be punished. When he realized that he had gone beyond the movement’s line, he backtracked.

2022-24

Since the Dobbs ruling overturning Roe, with abortion rights now more of a motivator for supporters than opponents, Mr. Trump has tried to reinvent himself again.

He now says he wouldn’t sign a federal abortion ban, though as president he endorsed a ban on abortions after 20 weeks of pregnancy. He also suggested that he might vote for a ballot measure in Florida that would increase abortion access, then said he opposed it — part of a series of politically motivated contortions.