Who is the candidate of change?

Joe Biden made Donald Trump feel fresh and vital.

No matter how commonplace Trump's tropes and mode of campaigning had become, they seemed compelling compared to the bleached-out president of the United of States who had become a shell of himself.

With the Trump-Biden contrast no longer relevant, the former president is operating in a new, much less forgiving environment. Kamala Harris wants to run a youth-vs.-age and future-vs.-past campaign against Trump, and she has some chance of making it work.

Against Biden, Trump represented the past, but also change. Against Harris, he's potentially just the past.

It's not "old" as a matter of age that's the issue, although all those concerns are now about Trump. Ronald Reagan was old when he took office, but was offering a complete change of direction in policy and exuded a youthful optimism and self-confident patriotism. The problem for Trump is "old" as a matter of feeling familiar, tired and played out.

The Mar-a-Lago press conference last week was a typical, nay, stereotypical, Trump event — Trump looked commanding against a vivid backdrop of American flags, but how many times have we seen that image?

He was a bit of everything — on message and off message, confident and defensive, charming and insulting, and so it went. Again, how many times have we watched it?

Even Trump's outrages aren't that surprising. That he went with the "Kamala suddenly became Black" line of attack wasn't exactly predictable, but nor is this kind of thing unexpected.

And, of course, we've repeatedly experienced cycles of hope for a new, more disciplined candidate dashed by Trump's insistence on doing it his way.

Again, none of this mattered so much against a doddering 81-year-old man who a vast majority of the public thought incapable of serving another four years. Biden was the past in everything he said and did.

For her part, Kamala Harris may not really be hip, but she is hipper than Trump. She's certainly energetic enough for a full slate of campaigning, and she's presenting herself as a third force: neither Biden nor Trump, a politician with an entirely new "vibe."

Harris has another advantage. It wasn't truly possible to cover up Biden's weakness. Even if Biden wasn't doing many interviews, he had to be out in public — at international meetings, at White House events and the like. No matter how much the Democrats insisted everything was OK, he could be seen stumbling, wandering and losing his train of thought.

With Harris, Republicans might (for good reason) say that she will lose herself in word-salad incoherence upon her first contact with a challenging interview, but there is no way to establish it without such an interview.

On the teleprompter, she seems just fine. She's pointed, amusing, determined and lifted by enthusiastic crowds.

Most importantly, Trump was winning a change election against Joe Biden. Now, he's essentially tied with Harris on who will bring positive change. The new CNBC poll had Harris at 39% on this question and Trump at 38%.

There is plenty for Trump to work with to pull ahead on this metric. People remember his record in office more or less fondly, and Harris has been an integral part of a failed administration and now embraces almost all of Biden's policies.

This isn't a case that makes itself, though. It's not enough simply not to be Kamala Harris, the way it was not to be Joe Biden.

Trump is going to have to make focused attacks that break through and aren't lost in the haze of pointless controversies. This presents a tactical question: If the choice is between an overly controlled candidate who is relentlessly on message and an ill-disciplined candidate who is off message, is it clear that the former (Harris) is worse than the latter (Trump)?

Trump has a new challenge — his opponent is no longer an aged incumbent president who has worn out his welcome.