The plan is "not a health care bill," Obama declared in a 939-word message to his nearly 53 million followers on Facebook. "It's a massive transfer of wealth from middle-class and poor families to the richest people in America."
The 44th president did not mention his successor, Donald Trump, but his scathing criticism and urgent tone - imploring his supporters to speak out against the "fundamental meanness at the core of this legislation" - set up a direct public fight with the current White House occupant over the future of the nation's health care system.
"I am very supportive of the Senate #HealthcareBill," Trump wrote in a tweet a short while later. "Look forward to making it really special! Remember, ObamaCare is dead."
The high-stakes confrontation is virtually unprecedented in modern times between a former and current president, and it runs counter to Obama's own professed interest in receding from the limelight. Just days before departing the White House, he joked that he looked forward to not hearing himself "talk so darn much."
Beyond his self-deprecation, Obama explained that he wanted to afford respect to Trump to pursue his own agenda, citing the precedent set by George W. Bush's infrequent public statements after Obama took office in 2009. Instead, since Trump's inauguration, Obama has made clear that he does not intend to stay on the sidelines as Trump, with help from Republican lawmakers, seeks to dismantle his legacy.
Obama spoke out in January after Trump implemented a travel ban on citizens of seven majority-Muslim nations, declaring that "American values are at stake" and that he was "heartened" by protests across the country. This month, Obama criticized Trump's decision to withdraw the United States from the Paris climate accord that his administration signed in 2015, ruing "an absence of American leadership."
But it is on health care that Obama has perhaps the most to lose and, with his lengthy Facebook statement, has signaled his intention to have the most political influence. Though he opened his message with an attempt to elevate the debate - emphasizing the need to listen to those with opposing points of view - he quickly framed Republican motivations as purely partisan.
"I recognize that repealing and replacing the Affordable Care Act has become a core tenet of the Republican Party," Obama wrote, suggesting that the GOP is acting simply to undo "something that Democrats did." His mention of "meanness" in the Senate bill was a swipe at Trump having called the House version of the repeal legislation "mean" during a private meeting with Republican senators last week.
The fight over the Affordable Care Act, the former president's biggest legislative victory, has sharply divided the two major political parties from the start. The bill was approved by Congress without a single Republican vote, after which the GOP successfully used it as a campaign issue against Democrats in the 2010 midterms that led to Republicans taking control of the House.
Now the tables have turned as Republicans attempt to make good on their years-long pledge to overturn the law. House Republicans needed two attempts before they successfully crafted and approved their own repeal bill without bipartisan support.
In recent weeks, Trump has lambasted Democrats as standing in the way.
At a campaign-style rally in Cedar Rapids, Iowa, on Wednesday, Trump declared that "Obamacare is a disaster" and added that "if we went and got the single greatest health care plan in the history of the world, we would not get one Democrat vote because they're obstructionists."
Democrats defended Obama's decision to wade into the political fight by accusing Trump of leveling personal attacks against him on a regular basis. In March, Trump, with no evidence, erroneously accused Obama of ordering a wiretap on Trump Tower in New York - an Obama spokesman called the accusation "simply false."
And this week, Trump indirectly criticized Obama for not doing more to secure the release of American college student Otto Warmbier, who died in Cincinnati days after returning home after being detained 17 months in North Korea.
"Donald Trump has invited Barack Obama into the arena," said Simon Rosenberg, founder of NDN, a liberal think tank. "No president has trashed a former president more than Trump has trashed Obama - personally and in terms of his legacy. It's been direct, persistent and out of bounds."
Obama is "obligated" to weigh in, Rosenberg said. "I don't think he wanted to play a major role or to get intimately involved. But it's become very personal. There comes a point where you can't stand aside."
Obama's public influence remains undeniable. Within three hours, his Facebook message had garnered more than 300,000 "likes" and 97,000 shares.
At the same time, his prominence in the debate highlights a dilemma for Democrats who are undergoing a painful search for a galvanizing agenda and new party leader in the wake of Hillary Clinton's loss to Trump last November.
After Democrat Jon Ossoff's loss in the Georgia special election for a vacant House seat this week, Republicans, including Trump, gloated that they hoped House Minority Leader Nancy Pelosi, D-Calif., and Senate Minority Leader Charles Schumer, D-N.Y., would remain in their jobs, citing their unpopularity with some moderate voters.
"I certainly hope the Democrats do not force Nancy P out," Trump wrote in a tweet. "That would be very bad for the Republican Party - and please let Cryin' Chuck stay!"
The bad news for Trump is that his own job approval ratings have plunged below 40 percent in some recent polls in the wake of his struggles to move forward with his agenda and an ongoing FBI investigation into his campaign's contact with Russian operatives.
By comparison, Obama's approval ratings this month stood at 63 percent, according to Gallup. For that matter, George W. Bush, who left office with just 35 percent of the public supporting him, was at 59 percent approval in the same Gallup survey.
"My guess is that part of the urgency of him weighing in is that a vote is happening within a week," Rosenberg said of Obama. "Trump and the Republicans are drawing him in and it's to their own detriment in doing so. He's still arguably the most popular and potent political force in either party and he can have a big impact."