Kari Lake, a former TV news anchor who refashioned herself as an ally and protégé of Donald J. Trump, officially announced she was running for the Senate in Arizona on Tuesday, offering a dark vision of America and pitching herself as a fighter ready to take on the media and the left.

Ms. Lake, who unsuccessfully ran for governor last year and never conceded her loss, mentioned that past election only once. She instead used her announcement speech to lavish praise on Mr. Trump, who endorsed her and called her “one of the toughest fighters in our movement” in a short video aired at the rally.

She took shots at President Biden and her potential rivals and stoked fear about Democratic policies that she said had led to rising inflation, high mortgage rates and what she repeatedly called an “invasion” at the nation’s southern border.

“I am not going to retreat,” she said at a rally held inside an airplane hangar of a luxury and private jet magazine. “I am going to stand on top of this hill with every single one of you.”

Ms. Lake, 54, is running for the seat held by Senator Kyrsten Sinema, who dropped her Democratic affiliation last year to become an independent. Ms. Sinema hasn’t announced whether she will run for re-election, but the race already includes Representative Ruben Gallego, a Democrat.

Ms. Lake has been teasing a Senate run for months, and filed paperwork to open her Senate campaign last week.

Flashing her usual brand of no-holds-barred politics on Tuesday, she described Ms. Sinema and Mr. Gallego as “Joe Biden enablers,” and said the president had sent the United States on a “kamikaze mission of total destruction.” She added, to cheers: “You Joe, you Joe — are a threat to America.”

But she also sought to strike a slightly different tone, pledging to work for both Democrats and Republicans and casting herself as a Republican with a vision for the future. She promised to restore factories, end the “urban camping” that results from homelessness and restart oil and gas drilling in the state. She pledged to fight for integrity in elections, a subtle reference to her unsuccessful legal challenges of her 2022 loss. And she argued that “the Republican Party should be about helping women and saving babies.”

She described herself as “the only mom in the race,” adding, “Every decision I make is going to be made with our children in mind, with our families in mind.”

Her most bombastic attacks were on the border, which she said was “owned” by the cartels. “Let’s start treating these cartels like the terrorists they are,” she said.

The Senate race in Arizona is expected to be one of the most closely watched in the nation. The state, along with West Virginia, Montana and Ohio, are seen as among the best opportunities for Republicans to pick up Senate seats next year and win back a majority. Ms. Lake, Ms. Sinema and Mr. Gallego could potentially find themselves in a three-way race pursuing their own slices of the electorate in what has become a national battleground.

“Arizonans know exactly who Kari Lake is — and that’s why they rejected her the first time around,” Mr. Gallego said in a statement after the rally, adding that she is running the “same, tired playbook of undermining our democracy.”

Mr. Trump’s endorsement came weeks after he told a prospective Republican candidate, Blake Masters, who lost a race for the other Senate seat in the state last year, that he would lose the primary against Ms. Lake if he ran, according to people familiar with their conversation.

Ms. Lake embraced the former president’s lies about winning the 2020 election in her failed run for Arizona governor last year. He pointed to her as an example of how to win, in a conversation between Mr. Trump and Mr. Masters that was recorded by a television camera trailing Mr. Masters.

A spokesman for Mr. Trump did not respond to a request for comment on Tuesday evening.

On Tuesday, a crowd of about a couple hundred people decked in American flag gear and “Save America” red hats chanted her name as she took the stage. Some waved yellow placards with her photo molded in the image of Rosie the Riveter.

In the crowd were Barbara Wellington, 63, a former small business owner, and Cindy Finnegan, 65, a retired speech pathologist, who drove in from nearby Goodyear, Ariz.

“I like her, I like who she supports,” Ms. Finnegan said, referring to Mr. Trump.

“She doesn’t give up,” Ms. Wellington said.

Jonathan Swan, Maggie Haberman and Michael C. Bender contributed reporting.