Michael Bloomberg, the billionaire former New York mayor and champion of causes such as gun control and climate change, has ended months of speculation by announcing that he will not run for US president next year.

The 77-year-old, one of the richest men in the world, revealed his decision on Tuesday not to join the crowded Democratic field – currently at 16 candidates – but pledged to pour his vast wealth into the resistance movement against Donald Trump.

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

Bloomberg has toyed with a presidential run before as an independent but re-registered as a Democrat last year and spent recent months courting primary voters as a political centrist. However, the fiscal conservative increasingly seemed out of step with a liberal base questioning the morality of economic inequality and the influence of money in politics.

Another billionaire, Tom Steyer, who has been campaigning for Trump’s impeachment, read the runes and decided in January not to make a bid for the White House. It appears that Bloomberg has reached a similar conclusion. His pro-business positions and support for stop-and-frisk policing as mayor of New York would also have come under scrutiny.

Neil Sroka, spokesperson for the progressive political action committee Democracy for America, said: “No surprise. No constituency. He had an additional challenge as a billionaire at a moment when the last thing people want is someone with ungodly sums of money telling them what to do. More importantly, he was pushing an agenda that has little support in the Democratic party and even less outside the Democratic party.”

There will not, after all, be a battle of the septuagenarian New York billionaire businessmen, between Trump and Bloomberg. Sroka added: “It shows Bloomberg has one thing over Trump: he’s not so blindly narcissistic that he got into this race.”

In the editorial, Bloomberg urged Democrats to unify behind a nominee who could defeat Trump, an implied warning against veering to the left as some candidates embrace a Green New Deal and Medicare for All – already painted by Republicans as socialist projects.

Senator Bernie Sanders, a self-declared democratic socialist, is among the early frontrunners. Howard Schultz, the billionaire former chief executive of Starbucks, is considering a run as an independent despite a fierce backlash from voters who fear he will peel off Democratic votes and help re-elect Trump.

Following Bloomberg’s announcement, Schultz tweeted: “I’ve long said there isn’t room for centrist moderation in either party and it appears Mr Bloomberg has come to the same conclusion.”

.@MikeBloomberg governed from the center with big ideas, pragmatism and common sense. In an era of paralysis and dysfunction, he’s an exception. I've long said there isn't room for centrist moderation in either party and it appears Mr. Bloomberg has come to the same conclusion. — Howard Schultz (@HowardSchultz) March 5, 2019

However, Bloomberg may be pinning his hopes on Joe Biden, the former vice-president, joining the fray as a centrist heavyweight.

“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, the UN secretary general’s special envoy for climate action, is expected to play an active role in the 2020 election from the outside. In his editorial, he said he would focus on tackling climate change and addressing gun violence, noting that both still face major obstacles in Congress.

This will include “Beyond Carbon”, a grassroots effort to begin moving America as quickly as possible away from oil and gas and toward a 100%-clean-energy economy.

Bloomberg spent more than $100m to help Democrats in the 2018 midterm elections, and his team has been preparing a data-driven effort to go much further in 2020. While the effort would have supported Bloomberg’s presidential bid had he run, it will now be used to defeat Republicans.

Chuck Schumer, the Democratic minority leader in the Senate, told the New York Times: “Mike Bloomberg was a great mayor and would have been a strong candidate. I am certain his passion for making the country a better place will continue, albeit not as a candidate, particularly on issues like climate change, guns and curtailing the abuses of President Trump.”

Bloomberg’s announcement followed confirmation that Hillary Clinton, the former secretary of state and 2016 Democratic nominee, will not run again next year. Trump tweeted in response: “‘(Crooked) Hillary Clinton confirms she will not run in 2020, rules out a third bid for White House.’ Aw-shucks, does that mean I won’t get to run against her again? She will be sorely missed!””