On Jan. 30, I was walking in New York when I felt my phone vibrate. A news alert said that Rep. Henry Waxman (D-Calif.) had announced he would not run for reelection. I stopped in my tracks. Despite the cold day, my face suddenly felt warm.

I’ve lived in Waxman’s district, on the west side of Los Angeles, for 18 years. I’ve always thought that when Waxman stepped down I would consider running for office. I believe in public service, and I worked in the Clinton White House from 1993 to 1995. I’d flirted with the idea of politics back then. When we left the White House, my girlfriend, Jody, and I were about to get engaged. She had worked for Bill Clinton for about the same time. I told her I was thinking about running.

“You can do that,” she said. “Or you can marry me. You can’t do both.”

So I didn’t. It was the right choice. We married, moved to L.A. and built a life filled with blessings in Pacific Palisades. Our 16-year-old is a California girl. I grew comfortable with the idea that I could contribute to public life through books and journalism and by hosting what’s become a popular program on public radio.

Still, the idea of elected office stayed in the back of my mind.

The district Waxman has represented for 40 years is a beautiful corner of the world that includes Malibu, Calabasas, Pacific Palisades, Bel Air, Beverly Hills and Santa Monica and stretches down along the coast to the South Bay and Palos Verdes. It’s among the wealthiest and most politically engaged constituencies in the country. It seemed likely that whoever succeeded Waxman would be able to keep this safe Democratic seat for as long as he or she wished to serve. So if I were ever going to seek the honor of representing my neighbors, it was now or never.

The timing seemed fated to test my values. The reason I was in New York when the Waxman news broke was to meet with a publisher about a book proposal. Its working title is “Making Victory Matter: Why Democrats Need to Think Big Again.” The premise is that, after eight years of a talented president whom much of the country sees as a “socialist,” virtually every measure of a good society that progressives care about (save for expanded health coverage) will be going in the wrong direction.

Whether it’s health costs, inequality, stagnant wages or lagging schools, global competition and rapid technological change have battered the middle class — and swamped the puny steps Democrats typically offer in response. Yet Democrats increasingly find themselves in a seductive trap, able to win national elections (thanks to “on your side” appeals and changing demographics) while doing little to improve the conditions or prospects of average Americans. For all the party’s rhetorical commitment to “equal opportunity,” “economic security” and “upward mobility,” what voters actually get from Democrats these days is a kinder, gentler decline.

That can’t be good enough. And that’s what I thought standing in the cold on 58th Street. I could write another book. Or, with Waxman leaving, I could enter the arena and try to help change the country’s direction.

The whirlwind days I’ve spent assessing the race have given me a lot to think about. It’s one thing to decry the money chase in theory, for example, and quite another to plan for the five hours a day you’ll spend courting wealthy donors on the phone.

Then there’s the matter of who votes. One consultant handed me a page showing that half of the primary voters will be older than 65! Voters age 18 to 34 are a rounding error. I’ve written for years that there’s no constituency for the future. But there’s nothing like seeing it through a candidate’s eyes.

There is also, of course, the toll on one’s family.

So why do it? Blame Paul Ryan. Watching the Wisconsin Republican move a hollow, regressive “plan” to the center of the national conversation in recent years made me realize what a void there is in Washington for anything resembling fresh ideas. The search for solutions equal to the scale of our challenges has been my stock-in-trade in journalism; it’s time to get such ideas off the sidelines and onto the field.

There is also Waxman’s remarkable example. I like to think the campaign I’m launching would be the kind he would run if it were his first race and the country faced its current challenges. As I’m learning, the path to get the chance to help move the nation forward is downright crazy. But it’s the only path there is.

So I’m in. The primary is June 3. Wish me luck.

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Matt Miller, a senior fellow at the Center for American Progress and host of the public radio program “Left, Right and Center,” has written a weekly online column for The Post, which now goes on hiatus. His e-mail address is mattmillernow@gmail.com.