Former New York City Mayor and presidential hopeful Mike Bloomberg (center) laughs with Portland Mayor Kate Snyder and former U.S. Rep. Mike Michaud during a photo op inside Becky's Diner on Commercial Street in Portland on Monday. Bloomberg was in Maine to officially open a campaign office in Scarborough.

SCARBOROUGH, Maine — Former New York City Mayor Mike Bloomberg, who was in Maine on Monday to plug his high-dollar Democratic presidential run, said he would actively back his party’s challenger to U.S. Sen. Susan Collins despite past donations to her.

Bloomberg, who first won election as New York’s mayor as a Republican in the early 2000s and became a Democrat in 2018, was a supporter of Maine’s Republican senator, giving her maximum donations during her 2008 and 2014 re-election races. After her 2018 vote for U.S. Supreme Court Justice Brett Kavanaugh, Collins is one of Democrats’ top targets in 2020.

Bloomberg, a media executive whose net worth is estimated by Forbes at nearly $60 billion, was a late entrant to the Democratic primary field, announcing his candidacy in November. He isn’t contesting the early-voting states of Iowa, New Hampshire and Nevada, instead focusing heavily on a dozen other states, including Maine, that will be holding March 3 primaries.

He was in Maine on Monday to open a Scarborough campaign office and for a photo op at Becky’s Diner in Portland. In a Monday interview, Bloomberg said he would devote resources to supporting Democrats in the state — including a Collins challenger — regardless of whether he wins the nomination to face President Donald Trump.

The nonprofit that Bloomberg founded, Everytown for Gun Safety, poured more than $6.2 million into supporting a Maine referendum in 2016 that aimed to expand background checks to include private gun sales and transfers, but failed. He said he backed Collins in previous elections because of her general support for expanding background checks.

“[It] was a tough thing for her to do, and I wanted to say thank you,” Bloomberg said, adding that he would back the Democratic nominee in 2020.

Guns remain a part of the former New York City mayor’s campaign, with his platform calling for tightening background check requirements and banning assault weapons and high-capacity magazines. Republicans protested outside of his new office Monday afternoon.

Bloomberg has put issues including health care and climate change forward in this run. He said he felt compelled to jump into the Democratic primary after initially saying he wouldn’t because he was not convinced that any of the other candidates could defeat Trump.

“On health care and every other issue, what you need is a president who will listen to you and deliver for you,” he said in a speech in Scarborough. “That’s what I’ll be and that starts with winning Maine.”

Bloomberg, who is self-funding his campaign, has already spent more than $250 million on television ads nationwide, including at least $1.4 million of the $2.1 million in ads booked by presidential candidates so far in Maine, according to FiveThirtyEight.

He is one of 10 active candidates who will be on the ballot for the Maine primary, which is highly uncertain. Different surveys in October showed Massachusetts Sen. Elizabeth Warren with a lead and Vice President Joe Biden with one. Vermont Sen. Bernie Sanders, who won the Maine caucuses in 2016, has gained ground in national polls since then. Bloomberg is in fourth place behind those candidates in national polls aggregated by RealClearPolitics.

Attendees said they were still deciding on who they would support in the Democratic primary but were interested in hearing from Bloomberg, though a few were skeptical of how his stance on guns would play in Maine.

M.D. Mitchell of Scarborough said he would certainly vote for Bloomberg against Trump, adding that he liked the mayor was self-funding and not beholden to anyone. But he was still unsure about who he would vote for in the primary since a few of his favorites have dropped out.

“We’ll see who is still here by March,” he said.