Mike Mikus remembers it as a “kick in the gut.”

Amid one of the most pivotal campaigns in the country in 2016, one many thought could decide control of the Senate, Mike Bloomberg poured millions of dollars into the contest — to help Pennsylvania Republican Sen. Pat Toomey.

“It helped both operationally in terms of the money spent, but it also allowed Pat Toomey to use that endorsement to create a false impression that he’s a moderate,” said Mikus, a consultant who worked for Toomey’s Democratic opponent, Katie McGinty.

Bloomberg, now a rising Democratic candidate for president, used his television ads to vouch for Toomey as a centrist problem solver. Toomey was one of the few Republicans in Congress to support expanded background checks for gun purchases, an issue on which Bloomberg has backed candidates in both parties.

It was a critical endorsement in a race Toomey won by just 1.5 percentage points, helping Republicans keep a narrow three-vote edge in the Senate. Much of the difference came in the moderate Philadelphia suburbs, where Toomey pitched himself as an “independent voice” — and where Bloomberg’s ads played frequently.

“He outperformed Trump in the Philly suburbs, and I believe the entire margin was made up right in those counties,” Mikus said.

A new national poll for NPR, PBS NewsHour, and Marist, released Tuesday, showed Bloomberg with 19% support among Democrats and independents who lean Democratic — second to Sen. Bernie Sanders, who had 31%.

Bloomberg’s group Independence USA spent almost $6 million on ads supporting Toomey in the Philadelphia media market four years ago.

The Inquirer first noted the endorsement’s potential consequences for Bloomberg in November, as the billionaire former New York mayor, who was first elected in 2001 there as a Republican, prepared to enter the Democratic fray.

But the topic is drawing added attention now that Bloomberg is gaining traction in Pennsylvania and nationally, and it could arise Wednesday in the next Democratic debate. Bloomberg will join the debate stage for the first time and directly confront his Democratic rivals, who have increasingly attacked his record as out-of-step with the party.

Bloomberg was a political independent at the time he helped Toomey, and earlier that year, there had been speculation he’d run a third-party campaign for president. In 2007, the midpoint of his second term as mayor, Bloomberg left the GOP and registered as an independent. He became a Democrat in October 2018.

In Pennsylvania, Toomey has become a lightning rod for liberal activists, who are likely to play a major role in the state’s April 28 primary.

Bloomberg backed Toomey because of his stand on an issue that is close to Democrats’ hearts: tougher gun laws. Toomey had co-sponsored a bill to expand background checks. Several other gun control groups also endorsed Toomey, including one led by former Democratic Rep. Gabrielle Giffords, who was gravely wounded in a 2011 Arizona shooting.

“Mike Bloomberg’s record on gun violence prevention speaks for itself," said Mustafa Rashed, a spokesperson for Bloomberg’s campaign in Pennsylvania. "He built a nationwide movement to take on the NRA and Trump and won stronger gun laws in states across the country, and he worked across the aisle to get it done. Mike’s record of supporting Democrats is equally strong — in 2018 he played a key role in helping Democrats retake the House as he invested in more than 20 successful seat flips, including Pennsylvania’s 6th District. As president, he will unite and rebuild our country at a time when it is more divided than ever.”

Democrats note that Toomey’s opponent, McGinty, supported far stronger gun measures, including bans on assault-style weapons. And in the reality of the Senate, the Republican majority bolstered by Toomey’s victory meant gun bills, and a host of other liberal priorities, were doomed. None would see any light as long as Majority Leader Mitch McConnell (R., Ky.) controlled the chamber.

“The Senate was up for grabs, and he clearly sided with Mitch McConnell,” Mikus said of Bloomberg.

Bloomberg, in announcing his Toomey endorsement in August 2016, said, “Chances are that no candidate is ever going to be perfect for you on every issue, but when brave people buck party orthodoxy and demonstrate bold, pragmatic leadership, they deserve support from all of us."

Bob Brady, chairman of the Philadelphia Democratic Party and a Joe Biden supporter, has in the past been critical of Bloomberg for his support of Toomey.

“I don’t think there’s any flavor for him here,” Brady said in November. “He’s for Republicans. We’re not for Toomey, naturally.”

But reflecting the uncertainty of the Democratic nominating contest, Brady sang a different tune in January, calling Bloomberg “probably” his second choice.

Reached Tuesday, Brady said: “Beating Trump will overcome all that, no matter who the candidate [will] be. Everybody’s got baggage.”