A crowd fills streets of Washington during the Women’s March on Jan. 21. (Oliver Contreras for The Washington Post)
Opinion writer

The nation’s streets were filled with anti-Trump opposition Saturday. This passion could be channeled into a movement that is reasonable and effective. Or not.

Untold hundreds of thousands of people swamped city streets from Washington, D.C., to Washington state. It was called the Women’s March, and when the crowd, a sea of pink knit hats, cheered along Pennsylvania Ave., the roar was markedly high-pitched. But even before the march began, it had transformed from a feminist rally into a larger political statement, drawing men and women, longtime activists and first-time protesters, and signs about health care, climate change, political civility and other big causes. Neither the media nor Democratic astro-turf groups manufactured this opposition. It is real, legitimate and wasting no time.

If it is to make a difference, it cannot seem to be merely a knee-jerk reaction to a Republican victory or another excuse for the same old activists to beat the same old drums. By rallying en masse before President Trump has had a chance to govern, the protesters risked seeming ungracious. This criticism would have been more potent if Trump had not delivered the inaugural address he did Friday, ominously claiming to be the embodiment of “the people” and implying that those who disagree with him are something else. Trump did not earn a honeymoon.

The movement will nevertheless fail if it is about one pet cause, or even a collection of pet causes. That approach did not work for Hillary Clinton.

It was unhelpful when Michael Moore used Saturday’s rally to press his views on internal Democratic Party politics, plugging the candidacy of Rep. Keith Ellison (D-Minn.) to be Democratic National Committee chairman. It was counterproductive when Madonna dropped the F-bomb on the rally’s critics and talked about blowing up the White House, eliciting some groans from the crowd. The movement should be more than an emotional outburst and bigger than just defending Planned Parenthood or supporting an Elizabeth Warren presidential bid. The appeal must be rooted in broad-based concern about an unpopular new president who is leading the country into dangerous, divisive political territory.

The seeds of a mature approach were there Saturday. Despite Trump’s graceless inaugural speech, actress Scarlett Johansson extended him a hand. “I didn’t vote for you,” she said, “but I want to be able to support you. But first I ask that you support me.” Liberal commentator Van Jones told conservatives they could do better than Trump — and he offered words of conciliation: “I’m tired of hearing us say ‘love trumps hate,’ but sometimes sound more hateful than Trump,” he said. “I’m tired of us — and I’ve been guilty of it — putting down all the Red State voters, and saying that they’re all stupid and they’re all uneducated. We have to stop that.”

These speakers did Trump supporters the courtesy that Trump failed to extend his detractors on Friday: They reached out to their fellow Americans, because the national debate has become too unreasonable.

The next step is organizing around issues that should appeal to a wide swath of the country during the Trump presidency: Dignity in the White House, clean and competent government, climate change, the hasty Obamacare repeal, respect for women, reverence for essential freedoms and income inequality, to name just a few.

It might be hard to imagine massive crowds fired up in the streets marching for reasonableness. But that is what the country needs.