Former New York City mayor Michael Bloomberg is officially a Democrat again.

The global media company founder said he has registered as a Democrat, which would be especially significant if he decides to challenge US President Donald Trump in 2020.

Bloomberg says on social media he did so "because we need Democrats to provide the checks and balance our nation so badly needs."

Bloomberg did not say when he might make a decision on running for president.

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He served three terms as New York City mayor and has variously been a Democrat, a Republican and an independent.

He twice flirted with running for president as an independent candidate.

Bloomberg has thrown money and support behind Democrats and is attacking Republicans on abortion and gun policies.

Given his wealth, Mr Bloomberg will not have to rely on raising money for his campaign, nor will be beholden to financial donors, meaning he could be a huge threat to Mr Trump.

A frequent critic of Trump, Bloomberg has donated $US80 million ($NZ123 million) to help Democrats gain control of the House of Representatives and $US20 million to help Democrats running for US Senate, which Republicans now narrowly control 51 to 49.

Democrats are aiming to pick up 23 seats in the House to win a majority in the lower chamber, something political analysts have said is more likely than gaining an edge in the Senate.

Taking control of either chamber would give Democrats leverage to oversee Trump and his administration, including the ability hold hearings and subpoena officials.

Bloomberg has used his fortune, which Forbes magazine has pegged at $US51.8 billion ($73 billion), to push a number of liberal causes including efforts to tackle global warming and gun deaths.

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- News.com.au