Former New York Mayor Michael Bloomberg is calling Boris Johnson's decisive victory in Britain's election the "canary in the coal mine" for the Democratic Party about its chances of unseating President Trump in 2020. Why it matters: The latest entry to the Democratic presidential primary field said Friday the U.K. results reinforce the idea that it's simply not enough for Democrats to assume they will beat Trump next year. That should push Democrats to have more concrete plans and policy ideas than they might otherwise come up with, Bloomberg told a small group of reporters and local climate volunteers in Alexandria, Va. What he's saying: "Maybe this is the canary in the coal mine. I think that beating Donald Trump is going to be more difficult after the U.K. election. That to me is pretty clear.""The public clearly wanted change in the U.K. and change that is much more rapid and greater magnitude than anyone predicted," he said. "I think it's sort of a catastrophic warning to the Democratic Party to have somebody that can beat Donald Trump and that is not going to be easy. Americans want to change, but I think they don't want revolutionary change — they want evolutionary change."Why you'll hear this again: Echoing Joe Biden's electability pitch, Bloomberg made the case that because of these rapid political changes and movements unfolding around the world, the country needs a Democratic nominee who doesn't need on-the-job training. "[Y]ou've got to step in, and right away, and put together the teams that you need to run the country."Our thought bubble: Democrats may well look to what happened in the "Labour heartlands" — where working class voters abandoned Jeremy Corbyn's Labour Party — as worrisome foreshadowing for the Rust Belt battles ahead, Axios' World Editor David Lawler offers. Some voters doubted that Corbyn's ambitious, left-wing platform was politically or fiscally possible. But some issues don't translate to 2020 in the U.S.Corbyn set records for unpopularity. Apart from his left-wing proposals, the party faced antisemitism crisis and there was general lack of confidence in him as a leader.Brexit cut across party lines and completely dominated this election. Johnson unified "Leave" backers and won a big majority with 44% of the vote, while the "Remain" vote was split. There's no equivalent issue in the U.S.Johnson actually made a pitch to the suburbs by ignoring social issues and focusing on things like the environment that are nowhere to be seen on Trump's agenda.Go deeper: U.K. election: Boris Johnson's big win means Brexit is comingThe U.K. election's warning for U.S. Democrats in 2020