PHILADELPHIA — Barack Obama’s mission on behalf of Hillary Clinton on Wednesday night was personal and political. He testified to her virtues as a would-be president in a way only a current president could. He insisted that the administration both of them helped fashion pulled the country from the economic abyss. And he sought to safeguard his legacy by ensuring his time in the White House would not be seen by history as having culminated in the election of Donald Trump.

And so he went to work, combining rational argument with evangelical exhortation in the classic Obama fashion and making clear that he saw only one logical choice this fall.

“And no matter how daunting the odds, no matter how much people try to knock her down, she never, ever quits,” he declared. “That’s the Hillary I know. That’s the Hillary I’ve come to admire. And that’s why I can say with confidence there has never been a man or a woman — not me, not Bill, nobody — more qualified than Hillary Clinton to serve as president of the United States of America.”

Obama’s witness on Clinton’s behalf reflected a profound change in our politics since a 2008 campaign during which Obama and Clinton represented very different views of how the system works and how change happens.

Back then, Obama promised that things could be done differently, that the divisions between red and blue were artificial, and that goodwill could prevail. Clinton warned that Obama’s view was unrealistic, once offering a biting satire of his optimism. “The sky will open, the light will come down, celestial choirs will be singing, and everyone will know we should do the right thing, and the world will be perfect,” she said, and added: “Maybe I’ve just lived a little long, but I have no illusions about how hard this is going to be.”

Both of them have now lived a little longer. Both have seen how the relentless oppositionism of the Republican Party led inexorably to Trump, the most dangerous and irresponsible nominee any major political party has ever put forward. Never before has a candidate asked a foreign power to conduct espionage on the United States, as Trump did Wednesday.

And that is how Obama saw the matter. “We are not a fragile or frightful people,” he insisted. “Our power doesn’t come from some self-declared savior promising that he alone can restore order. We don’t look to be ruled.” Trump, he said, “cozies up to Putin, praises Saddam Hussein, and tells the NATO allies that stood by our side after 9/11 that they have to pay up if they want our protection.”

Trump alone would justify Obama’s passionate intervention on Clinton’s behalf. But there is more: Both must grapple with an impatience embodied in Bernie Sanders’s campaign and a right-wing nationalism that disdains the quietly rational, socially tolerant, policy-oriented liberalism that animates their approach to governing. Differences that loomed large in 2008 masked what Obama and Clinton shared: Both were always evolutionary reformers. Both have always been about the politics of the possible.

Obama was unapologetic about this. “Through every victory and every setback,” he declared, “I’ve insisted that change is never easy, and never quick, that we wouldn’t meet all of our challenges in one term, or one presidency, or even in one lifetime.”

Obama spoke to the discontent in the country — “pockets of America that never recovered from factory closures; men who took pride in hard work and providing for their families who now feel forgotten.” But Trump’s apocalyptic view, he said, was a wild and dangerous invention, “a deeply pessimistic vision of a country where we turn against each other, and turn away from the rest of the world.” Trump’s Cleveland convention involved “no serious solutions to pressing problems — just the fanning of resentment, and blame, and anger, and hate.”

In the end, Obama argued, the changes the country actually wants are possible only under Clinton. They will not come from Trump. “Does anyone,” he asked, “really believe that a guy who’s spent his 70 years on this Earth showing no regard for working people is suddenly going to be your champion?”

Thus did a leader who won office promising “change we can believe in” turn to the task of defending continuity we can believe in. He made a compelling case that, compared with what a Trump presidency would look like, continuity is looking better by the day. “The America I know is decent and generous,” he said. It was a description. It was also a hope.

E.J. Dionne Jr. is a columnist with The Washington Post. His email address is: ejdionne@washpost.com. Twitter: @EJDionne.