For Sens. Kamala D. Harris and Cory A. Booker, Democratic presidential candidates who were promoting an anti-lynching bill, the “attack” on actor Jussie Smollett in Chicago looked like a horrible yet opportune talking point.

Instead, the apparent hoax is turning out to be a warning to 2020 Democratic candidates about singling out people for martyrdom, including the woman who accused Supreme Court Justice Brett M. Kavanaugh of sexual assault and the drum-banging American Indian who confronted Catholic high school students on the steps of the Lincoln Memorial.

In the rush to speak too soon about the Smollett case, Ms. Harris and Mr. Booker stand out.

Just before noon on Jan. 29, news reports emerged on social media that Mr. Smollett, an actor on the TV show “Empire,” had been attacked by two men yelling racial and homophobic slurs. He claimed the men said something about “MAGA country,” suggesting they were Trump supporters.

Mr. Smollett said the two men put a rope around his neck.

Five hours after the initial report, Ms. Harris, a former California attorney general, pronounced the attack genuine and called it “an attempted modern-day lynching.”

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“We must confront this hate,” she tweeted.

But this week, confronted with growing indications that Mr. Smollett’s story was a hoax concocted to help his television career, Ms. Harris has become a devotee of caution.

“I think that the facts are still unfolding,” she said. “I’m not going to comment until I know the outcome of the investigation.”

Two brothers told police that the “Empire” actor staged an attack on himself because he was upset that a threatening letter he reported a week earlier did not receive enough attention, a law enforcement official told ABC News. The FBI is looking into whether Mr. Smollett concocted the letter.

Mr. Booker, a New Jersey Democrat, also was initially an unquestioning believer in a lynching.

“To those in Congress who don’t feel the urgency to pass our Anti-Lynching bill designating lynching as a federal hate crime — I urge you to pay attention,” he tweeted hours after Mr. Smollett’s supposed attack came to light.

But this week, staring at the possibility that he swallowed a hoax gullibly, the senator said he would withhold judgment on the case until “all the information actually comes out from on-the-record sources.”

Donald Trump Jr. called Ms. Harris‘ newfound prudence “spineless spin.”

“Strange she was all over it when she thought it could serve a purpose,” the president’s son posted on Twitter. “Shocked that after jumping all over it when the narrative suited them now suddenly they won’t comment.”

He noted that even the Rev. Al Sharpton, who rose to prominence in the 1980s by defending a report of gang rape that was later discredited, called for “accountability” from Mr. Smollett after initially believing his story.

Mr. Sharpton said this week of Mr. Smollett and two friends, “If it is found that Smollett and these gentlemen did, in some way, perpetrate something that is not true, they ought to face accountability to the maximum. Let us get to the bottom of it, and let justice be done, no matter who is right or wrong.”

The president’s son commented, “Given the irony here of the past, it really says something that Sharpton is ahead of the pack of all the Democratic Presidential Hopefuls in condemning this terrible act of hate.”

Former U.N. Ambassador Nikki Haley said the episode couldn’t have taken hold in the public’s imagination without help from the mainstream media.

“The media should be the most outraged,” she said in a Twitter message. “[Mr. Smollett] played all of them for fools. He knew they would cover it.”

She said the actor “must be held accountable in the strictest way. He must repay resources used to investigate and serve time for the division he caused.”

President Trump initially spoke out against the supposed hate crime when reporters asked him about it Jan. 31.

“I can tell you that it’s horrible,” the president said. “It doesn’t get worse.”

The Senate unanimously passed legislation Dec. 19 that would make lynching a federal crime. Proposed by Mr. Booker, Ms. Harris and Sen. Tim Scott, South Carolina Republican, the measure classifies lynching as a hate crime and calls it “the ultimate expression of racism in the United States.”

The House has not approved the bill.

For many on the right, the Smollett case bears the pattern of media cooperation with liberal Democrats in promoting victimhood at the hands of conservatives. Christine Blasey Ford became a symbol of the #MeToo movement as she testified against then-Judge Kavanaugh this fall. Her uncorroborated story about a sexual assault in the 1980s gained acceptance from Democratic senators, who sought unsuccessfully to block his Supreme Court nomination.

Likewise, many on the left rushed to condemn a group of students from Covington Catholic High School in Kentucky who were videotaped in an encounter with drum-pounding American Indian Nathan Phillips. Rep. Ilhan Omar, Minnesota Democrat, tweeted that the boys were protesting a woman’s right to choose and taunting a group of blacks. She later deleted the tweet.

An investigation by private detectives for the Catholic Diocese of Covington found no wrongdoing by the boys.

Blogger Matt Walsh, a conservative who often comments on religion and culture, said the episode showed that the media are “not smart.”

“Journalists believed that psycho Trump fans were roving the streets of Chicago with a noose,” he tweeted. “They believed that a massive horde of white kids randomly accosted a Native American for no reason. They believed that a teenaged Brett Kavanaugh was part of an organized rape gang.”

Grammy-winning performer Cardi B, who was born as Belcalis Almanzar, said on Instagram that she is “really disappointed in Jussie Smollett.”

“I feel like he f–ed up Black History Month, bro,” she said.