Back in the days as a campaign flack, I learned that, while I was laboring over every word in a campaign speech or radio ad, most voters were way too busy raising kids and paying the bills to pay nearly as much attention as I thought they were.

Instead, a political veteran told me, the voters will reduce your candidate and his entire campaign down to one idea or concept. “You get one sentence, kid. Make it count.”

And so for Bernie Sanders, that one sentence is, “Socialist guy.” For Beto O’Rourke, it’s, “Kind of like a Kennedy, but weird.” For Pete Buttigieg, it’s the “Really smart, young … wait — he’s gay?” guy.

Liz Warren? “Fake Indian.”

On Tuesday I got to see former Vice President Joe Biden at a house party in Nashua, N.H. He gave a speech, he worked the crowd, he engaged in an inordinate amount of consensual hugging. And the one-word takeaway that was left, uncomfortably unspoken, on everyone’s lips was …

Old.

Joe Biden is “the old guy.”

I don’t mean Joe Biden was born Nov. 20, 1942, though he was. I don’t mean Joe Biden won his first U.S. Senate race while Richard Nixon was still president, though he did. I mean that Joe Biden’s age is a thing. It’s a front-and-center fact of his candidacy that is impossible to ignore.

And, I predict, it will eventually cost him his party’s nomination.

Karlyn Bowman of the American Enterprise Institute, who’s been analyzing polling data for four decades, told me that voters don’t care about age. “It doesn’t appear to make much of a difference, going back to 1940, when Thomas Dewey was just 37 years old and pollsters first asked the question,” she told me. But she and several political experts I spoke to agreed that the perception of Biden’s age is different from his septuagenarian colleagues.

After all, Biden supporters note, Bernie Sanders is almost a year older than Biden. Liz Warren turns 70 next month, and then there’s The Donald himself — no spring chicken himself at 72.

But these examples make the point. When you see Bernie Sanders, do you see “an old guy?” No. You just see, well, Bernie. He’s been ranting about workers controlling the means of production with that same “Larry David” look for decades.

As veteran Democratic consultant Bob Shrum told me, “Bernie’s always looked like Bernie. With Biden a lot will depend on how he comes across in the debates. Can he show, as Robert F. Kennedy said, that age is not a time of life but a state of mind?”

With Biden, it was a question the friendly crowd of New Hampshire Democrats was trying to avoid. “He seems vigorous,” one woman in the audience told me, which is the age-question equivalent of “she’s got a nice personality.”

At one point Biden was asked about the federal crime bill he helped craft and enact in 1994 and its disparate impact on communities of color. His long, meandering answer was incomprehensible. Now, that’s not necessarily a problem. Ask Liz Warren how she plans to pay for her $193 trillion(!) in new spending proposals and you’ll get a lot of “incomprehensible.”

What had the national media types around me glancing uncomfortably at each other was the way he didn’t answer. He wasn’t obstructing. He was struggling. Throughout the two-hour event, he would suddenly stop mid-sentence and careen off to another topic, like your grandpa’s running commentary while watching Fox News.

The headline at Axios is “Biden plots an early kill.” His campaign plans to run as though he’s already the nominee and facing Trump in the general election. Some people see this as a sign of Biden’s strength. It’s not.

Biden needs an “early kill” because running for president is hard work. This campaign cycle is the longest in American political history. The guy I saw in New Hampshire on Tuesday was not a guy who could spend the next nine months on the stump in Iowa and South Carolina.

“Joe has incredible energy, he has incredible passion,” his wife, Jill, told MSNBC. “I don’t think he’s too old … but the voters will make that decision when they see him out on the campaign trail.”

Unfortunately for Joe, I think she’s right.

Michael Graham is a regular contributor to the Boston Herald. Follow him at IAmMGraham on Twitter.