It’s one of this autumn’s pleasant surprises. Two years after Donald Trump’s election as president and 10 years after his own, Barack Obama is gracefully re-entering our consciousness, reminding us of what we have lost and may yet recover.

The contrast between Obama and Trump—decent vs. despicable; incisive vs. ignorant; honest vs. humbug; classy vs. clownish—is now the critical subtext of the 2018 campaign. With Obama’s current approval ratings more than 20 points higher than Trump’s, the aching memory of his presidency will help energize Democrats in the midterms.

But Obama’s return is also a reminder that some of his admirable qualities—modesty, prudence, deliberateness—have inadvertently helped Republicans endanger everything he built.

If Obama’s reputation is secure, his legacy is not. Many of his accomplishments in office are in danger of being wiped out in November. The personal stakes for him and his place in history are high.

As he campaigns around the country, Obama seems to have two immediate goals: First, to help elect Democrats up and down the ballot—to check Trump in Congress but also to rebuild the Democratic Party at the state level so that after the 2020 census it can undo some of the gerrymandering that has so often hindered his party.

Second, and related, Obama hopes to educate the public about the origins of our present ills, not just Trump lying about his birth certificate (which Obama still downplays) but a Republican Party that grew increasingly radical and obstructionist long before its new hero entered politics. And the former president wants to remind voters that the so-called “economic miracle” we keep hearing about is just another of what are now more than 5,000 confirmed lies by Trump since he took office; the economy today is creating roughly the same number of jobs than it did in the last two years of the Obama presidency—and fewer, in fact, than in 2014.

Obama’s arguments are welcome, but they raise the question of why he didn’t make them more aggressively when he was in office. Michelle Obama, whose post-election book tour means she won’t be stumping this fall for candidates who have invited her into their districts (she will work instead on voter registration and turnout), famously told the 2016 Democratic Convention, “When they go low, we go high.” In hindsight, this looks noble but a tad naive. When they went low, why didn’t her husband at least ridicule them, as he did so mercilessly to Trump at the 2011 White House Correspondents’ Dinner?

The answer is that as the first black president, he thought it was important not to sound strident or excessively partisan. And he didn’t want to demean his office and risk his reputation by descending to Trump’s level. Even now, he’s reluctant to mention Trump’s name, for fear of becoming a useful foil for the president. If Democrats fall short, historians may fasten on his reluctance to mix it up more when he had the chance.

To understand why so much is on the line this fall, it helps to briefly review Obama’s eight years in office:

His first year as president was historic. After the 2008 financial collapse threw millions out of work and threatened another Great Depression, Obama stabilized and then re-regulated (through Dodd-Frank) the banking system and—largely forgotten—also offered help for underwater homeowners. His $787-billion stimulus package and simultaneous bailout of the auto industry (which ended up costing the taxpayers nothing) revived the cratering economy. It included middle-class tax cuts and huge new spending on green energy, medical research, and scores of other important investments. And in early 2010 he achieved what had eluded every Democratic president since Harry Truman—universal health care coverage. Amazingly, all of this money was spent without scandal.

But when the Republicans took the House in 2010—the most fateful midterm election in the history of the Democratic Party—Obama’s ability to move legislation through Congress ended after only two years. From then on, just about all he could do on the domestic side was issue executive orders, which have been easy for Trump to reverse. The GOP united to block comprehensive immigration reform (an idea many conservatives had favored under Bush) and any new stimulus (including critical infrastructure spending) to hasten the recovery, which then—not coincidentally— remained anemic for four more years. Beyond that, Republicans developed a habit of reflexively rejecting everything the president proposed, even if they had recently favored it.

Obama’s key mistake after the Great Recession was to go along with Republicans and elite opinion when they prematurely changed the subject to deficit reduction. In 2011, the Tea Party, anticipating Trump, hijacked the GOP and moved it to crazy town by shutting down the government and nearly defaulting on the national debt—all in the name of fiscal responsibility they were faking, as proven by their unanimous vote in 2017 to slash taxes and balloon the deficit that Obama had cut in half.

Obama managed to get reelected in 2012 but he did so without engaging in the party-building activities necessary to retake the House. He had several international successes: killing Osama bin Laden, winding down wars in Iraq and Afghanistan, delaying Iran’s acquisition of nuclear weapons with a deal that worked, helping save millions of Africans from dying of AIDS and Ebola. Obama policies in Syria and Libya could be chalked up as failures, but the Trans-Pacific Partnership trade deal was a huge long-term advance for the American economy (not to mention an effective check on China) and the U.S. led the way in securing the Paris Accords on Climate Change, which eventually included every country in the world.