Michael Bloomberg is not running for president.

The 77-year-old former New York City mayor, one of the richest men in the world, announced his decision not to join the crowded Democratic field in a Bloomberg News editorial on Tuesday.

"I know we can do better as a country. And I believe I would defeat Donald Trump in a general election," he wrote. "But I am clear-eyed about the difficulty of winning the Democratic nomination in such a crowded field."

Bloomberg has flirted with a presidential run before, but as an independent. He registered as a Democrat last fall and spent recent months courting primary voters as a political centrist. But as an older white man with strong ties to Wall Street, he likely would have struggled to win over the Democratic Party's energized liberal base that's increasingly embracing diversity.

He encouraged Democrats to unify behind a nominee who could beat Trump, a not-so-subtle dig against candidates pushing the party to embrace liberal priorities such as "Medicare-for-all."

"It's essential that we nominate a Democrat who will be in the strongest position to defeat Donald Trump and bring our country back together," he wrote. "We cannot allow the primary process to drag the party to an extreme that would diminish our chances in the general election and translate into 'Four More Years.'"

Bloomberg is expected to play an active role in the 2020 election from the outside. In his editorial, he said he would focus on combating climate change and addressing gun violence.

Bloomberg invested more than $100 million to help Democrats in the 2018 midterm election; his team has quietly been preparing a data-driven effort to go much further in 2020. While the effort would have supported Bloomberg's presidential bid had he ran, it will now be used to help Democrats defeat Trump.