A big bet by “CBS Evening News” on Norah O’Donnell is beginning to look iffy.

The former “CBS This Morning” host — who is getting paid at least $7 million a year by CBS News boss Susan Zirinsky to revive the perennially third-ranked newscast — is getting hammered in the ratings worse than her predecessors did.

In the last week alone, O’Donnell’s total viewers plunged 17 percent to 5.1 million, according to Nielsen. Among viewers in the crucial demographic ages 25 to 54 that’s coveted by advertisers, the drop was a heart-stopping 25 percent.

The slide has steadily accelerated during the past month, with weekly ratings in the key age group falling between 20 and 29 percent versus a year ago. That’s far steeper than the single-digit drops during the year-and-a-half tenure of her predecessor Jeff Glor, who was pulling down $2 million a year, according to a source.

“This is definitely unusual,” said television analyst Andrew Tyndall. “I don’t see any prospects of ‘CBS Evening News’ getting out of third place, but an honorable third versus a dismal third is possible.”

It’s a troubling sign for CBS, whose evening newscast has been stuck in third place since the tail end of Dan Rather’s tenure, which ended in controversy over his reporting about President George W. Bush’s military service.

Asked about the decline, CBS sent The Post a statement from Zirinsky, saying the network is “extremely pleased with how quickly the ‘CBS Evening News’ with Norah O’Donnell has established itself as a must-see destination for exclusive reporting and investigative journalism. We’re just getting started — ratings are not the sole measure at this time.”

To be sure, all three nightly newscasts have gotten slammed as viewers increasingly defect to the web. In the 2018-19 season, ABC’s “World News Tonight” with David Muir came in first place, slipping 0.3 percent as it attracted 8.6 million viewers. On NBC “Nightly News,” Lester Holt’s total viewers fell 2.6 percent to 7.9 million, although he led the 25-to-54 group with 1.7 million.

But CBS’ dismal 2018-19 decline — in which its overall viewership sank 6 percent to 5.8 million through the middle of last month — has only worsened with the arrival of O’Donnell.

In a splashy July 15 debut tied to the 50th anniversary of the moon landing, O’Donnell interviewed Caroline Kennedy about her father, and Jeff Bezos about his space exploration ambitions. The show came in last place, with ratings off 5 percent from a year earlier.

Late last month, she bagged an interview with Saudi Crown Prince Mohammad bin Salman, in which he accepted responsibility for the killing of journalist Jamal Khash­oggi, but denied ordering the murder. It aired, however, on “60 Minutes,” not on her newscast.

A source close to CBS said top brass are giving the fresh-faced 45-year-old time to acclimate. They “didn’t think Glor could win,” and had a “strong belief she was the best they had for the job,” the source said.

Backers cite O’Donnell’s strong DC connections from her tenure as a White House correspondent, her experience reporting around the world and the fact that she “reads off the teleprompter well,” the source said.

Nevertheless, critics say CBS executives have botched the transition, failing to launch O’Donnell’s tenure with a major publicity campaign or slogan. The network has pledged to move O’Donnell to Washington — the first time a major news network has anchored its broadcast outside New York since the 1970s. But the timing has been vague.

“When she took over, they deliberately did what they do in the restaurant business — a soft open,” said Tyndall. “They said, ‘We are going to move her show to Washington, DC, but not yet. We are giving her a new producer, but not yet.’”

O’Donnell just landed a new executive producer in Jay Shaylor, who decamped from CNN’s “The Situation Room with Wolf Blitzer.” But insiders note ratings for Blitzer’s show haven’t been good, either.

Former TV executive Preston Beckman says viewers may be switching off O’Donnell because they are looking for “someone with more gravitas.” While she has decades of experience in the field, she’s green when it comes to anchoring a news desk, he said.

“A news anchor is different than a reporter,” Beckman said.

Another key factor: O’Donnell is broadcasting to an American public which, with the exception of Diane Sawyer and Katie Couric, has scant experience with female nightly news anchors since the start of TV.

“The obvious answer is she’s a woman,” Beckman said.

Tyndall, for his part, believes O’Donnell is “trying too hard.” He cited last Monday’s broadcast on gymnast Simone Biles and two other female athletes who won competitions over the weekend.

The segment was “introduced by a gushing” O’Donnell, who branded the segment “three stories of triumph,” calling them “so incredibly powerful.”