Mike Bloomberg is jumping into the Democratic presidential race because he believes that Joe Biden is fading, opening the moderate lane next to Elizabeth Warren, sources close to the former New York mayor tell Axios.

Why it matters: "Mike will spend whatever it takes to defeat Donald Trump," a Bloomberg source said. "The nation is about to see a very different campaign than we’ve ever seen before."

I'm told there's no way he'll later run as a third-party or independent candidate, partly because of ballot-access hurdles.

Theory of the case: Bloomberg, who according to Forbes is worth $52 billion, will self-fund, allowing him to run an essentially national campaign at a time when the rest of the field is raising money and focusing on early states.

Bloomberg, who will make a final decision "soon," isn't expected to seek or accept campaign contributions, according to a second source.

Bloomberg had been focused on how he could best influence 2020 from the outside. But he increasingly became concerned that all the leading Democrats have weaknesses Trump could exploit in the general election.

Bloomberg sees himself as an anti-Trump: practical and pragmatic, a self-made business leader, committed to issues such as climate and guns, and someone who recognizes the value of multilateralism and coalitions over isolationism.

What's next: The Bloomberg buzz ignited yesterday with the news that he'll file today to qualify for the primary in Alabama, which has an early filing deadline.

I'm told he'll quickly ramp up in other states with deadlines approaching, including Arkansas, New Hampshire, Florida, California and Texas.

Reality check: Given the progressive tides in the Democratic Party, there's no sign that a 77-year-old billionaire is what primary voters are pining for.

Biden is sucking wind on money, and now Bloomberg is moving into his lane with unlimited cash.

Go deeper: Michael Bloomberg is giving his friends a copy of his new biography