WASHINGTON (AP) — Michael Bloomberg, the billionaire former New York City mayor, announced Tuesday that he will not join the crowded field of Democrats running for president in 2020.

Instead, Bloomberg said he planned to focus his energy and considerable resources on outside efforts aimed at defeating President Donald Trump, as well as on combating climate change and addressing gun violence.

Bloomberg spent months weighing a White House run, traveling to early voting states and building a team of experienced political advisers. But aides said internal polling suggested Bloomberg’s path to the Democratic nomination was narrow, particularly if Vice President Joe Biden — who shares some of Bloomberg’s moderate positions — decides to run.

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In an editorial for Bloomberg News — the media company Bloomberg owns — he said he was “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 began pitching himself to primary voters as a political centrist. But as an older white man with strong ties to Wall Street, he may have struggled to win over the Democratic Party’s energized liberal base that’s increasingly embracing diversity.

He encouraged Democrats on Tuesday 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 aides said Biden’s likely White House run was a factor in the mayor’s decision. The team’s internal polling showed that there is an opportunity for a moderate, like Bloomberg or Biden, to win the Democratic primary, but there wasn’t room for both.

Biden may not announce his final decision until April. But Bloomberg concluded that was too long to wait to make his own decision, and he informed advisers on Monday that he would not be running for the White House.

Bloomberg does plan to keep his political network together as he considers how to play a role in the 2020 election from the outside. He’s working with several top advisers to former President Barack Obama, including David Plouffe, the architect of Obama’s 2008 campaign, data guru Dan Wagner and Mitch Stewart, Obama’s battlefield-states director.

Aides said Bloomberg believes Trump’s fundraising prowess is underestimated and the eventual Democratic nominee will need significant support from outside forces, particularly if the primary contest drags well into 2020.

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.