(CNN) Michael Bloomberg officially announced his late-entry Democratic presidential bid on Sunday, unveiling a campaign that the former New York mayor said will be squarely aimed at defeating President Donald Trump.

Bloomberg, in a letter explaining his candidacy on his campaign website , lays out a more moderate vision for the country and casts himself as "a doer and a problem solver -- not a talker."

"I'm running for president to defeat Donald Trump and rebuild America. We cannot afford four more years of President Trump's reckless and unethical actions," Bloomberg wrote.

Bloomberg's late 2020 bid -- along with the money the billionaire can spend to fund his campaign -- injects a new level of uncertainty into the race less than three months before the first voting in the race begins. In the last several days there was little doubt he was running.

Bloomberg, who had said earlier this year that he would not run, reversed his decision because he doesn't think there's a candidate in the current field of Democrats who can beat Trump next November, several people close to the former mayor told CNN. That includes former Vice President Joe Biden, who Bloomberg has watched fade in Iowa polling and struggle with fundraising.

Just hours after officially entering the 2020 Democratic race, Bloomberg was making calls throughout the day from New York, according to a person close to him.

Bloomberg is not the first late entry candidate to get into the race. Former Massachusetts Gov. Deval Patrick also announced earlier this month that he, too, would run for the Democratic nomination.

To kick start his campaign, Bloomberg has placed at least $37 million worth of television advertising over the next two weeks, according to data from Kantar Media/CMAG.

The ads highlight the mayor's biography -- "He could have just been the middle class kid ... but Mike Bloomberg became the guy who did good," said the ad -- and his post-mayoral work on combating climate change. Then the spot turns to Trump, saying now the mayor is "taking on him" as an image of Trump freezes on screen.

The spot ends with narrator saying "'Mike Bloomberg for President" with Bloomberg saying "I'm Mike Bloomberg and I approve this message."

Bloomberg's massive buy -- 60 second spots across some 100 markets -- will begin next week, representing more than the entire Democratic field has spent on TV advertising in the race so far, excluding businessman Tom Steyer, who will have aired nearly $63 million of TV ads by the end of Bloomberg's initial bookings.

A 77-year-old entrepreneur and philanthropist, Bloomberg made his fortune creating technology that bankers and traders use to access market data. After building a successful financial information business, he turned to politics. He officially launched a bid to become mayor of New York in 2001. Despite running as a Republican in an overwhelmingly Democratic city, Bloomberg won the election and was reelected twice. During this second term, he switched parties and became an independent -- only to re-register as a Democrat in 2018.

Because of his late entry, aides to the former mayor have said he won't compete in the first four voting contests, in Iowa, New Hampshire, Nevada and South Carolina. Instead, Bloomberg is staking his chances on an unconventional strategy of building support in the states that hold primaries on March 3, also known as Super Tuesday.

It's a strategy that has never been successful in Democratic presidential politics. Party officials in Iowa and New Hampshire have publicly expressed disappointment with Bloomberg's decision.

Bloomberg faces notable challenges

Bloomberg can tout his recent efforts advancing causes important to Democratic voters. He's put his significant financial resources behind efforts to defend reproductive rights and to tackle climate change and gun violence.

He spent more than $100 million to help the party take control of the House during the 2018 midterm elections and, more recently, contributed to important state races in Virginia.

But a Bloomberg candidacy could face several challenges, including countering the narrative that progressive candidates like Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts have already set: that billionaires shouldn't be able to "buy elections." It may also be difficult for Bloomberg to meet the polling and donor thresholds to make it onto debate stages.

The first opportunity for Bloomberg to make the debate stage will be in December, when those Democrats who meet the DNC's thresholds - receiving 4% in at least four national or early state polls that meet the DNC's criteria or 6% in two early state polls and receiving donations from at least 200,000 unique donors, with a minimum of 800 from at least 20 different states - gather in Los Angeles on December 19.

That could be a difficult climb for the former mayor, who only has until December 12 to reach the thresholds, but Steyer - who spent millions on digital ads to drum up the needed smaller dollar donations - proved it is possible.

This criticism of Bloomberg grew over the weekend when it became clear that Bloomberg was slated to spend tens of millions of dollars on ads at the outset of his campaign.

"I see this as one more example where how come when you have someone who is already a multimillionaire in the White House do you think that the people in this country are going to go, 'Oh, we need someone wealthier,'" Minnesota Sen. Amy Klobuchar said in New Hampshire on Saturday.

And California Sen. Kamala Harris turned Bloomberg's entry into a call for campaign finance reform. "Listen, we got to get money out of politics," she told an audience in Iowa. "I mean, I got to raise a ton of money to be competitive."

Then there's his past support for "stop and frisk," a type of aggressive policing that allowed -- critics say encouraged -- officers to detain a person on virtually any type of vague suspicion. After defending it for years, Bloomberg apologized last week at a predominantly African American church for implementing the controversial policy.

Some questioned the timing of his backtrack on the issue as he made moves toward a presidential run.

"It's interesting timing that the mayor would apologize for that now," said one of his rivals for the nomination, former Housing and Urban Development Secretary Julián Castro.

Bloomberg had flirted with jumping into the crowded contest for months in 2018 and early 2019. During that time, he crisscrossed the country, holding events in early-voting states that ranged from visiting factory workers in New Hampshire to hosting a premiere of his climate change documentary in Iowa.

It had looked like the beginning of a campaign. About a year ago, Bloomberg's longtime adviser Kevin Sheekey said they were building a team, including hiring former President Barack Obama's 2008 campaign manager, David Plouffe, among others.

But in March, Bloomberg said he wasn't running. Instead, the former mayor said he wanted to double down on his efforts to attack climate change and gun violence.

"I've come to realize that I'm less interested in talking than doing," he wrote in an opinion piece that was published in Bloomberg News.

This story is updated with additional developments.