This was Joe Biden’s first chance to define how he will be judged by history.

In a rare televised address from the Oval Office on Wednesday night, he spoke of his accomplishments. He spoke of his humble roots. He sang the praises of the American people. He said the future of American democracy lies in their hands.

What he didn’t do, despite saying he would always level with Americans, was provide a direct explanation for the biggest question of the day.

He didn’t say why he has become the first incumbent president to abandon a re-election bid, just a few months before voting begins.

And that is what the history books will be most interested in.

He hinted at it. He talked around it. But he never tackled it head on. It was left for the American people to read between the lines.

“In recent weeks,” Mr Biden said, “it’s become clear to me that I need to unite my party.”

He then echoed what has become a growing chorus among Democrats – that it was time to “pass the torch” to a new generation.

While he said his accomplishments, which he listed in detail, merited a second term in office, he added that “nothing can come in the way of saving our democracy – and that includes personal ambition.”

Left unsaid was the cold, hard reality that he resigned because it was becoming increasingly clear that he was going to lose to Donald Trump in November. And that is an outcome that those in his party universally view as catastrophic.

Trailing in the polls, embarrassed by a miserable debate performance and with a growing chorus in the Democratic Party calling for him to step aside, there was no clear path to a Biden victory.

While the president may not have said it, his Republican predecessor – and now former rival for the White House – had no such qualms.

At a rally in Charlotte, North Carolina, a few hours before the address, Donald Trump said Mr Biden dropped out because he was losing badly.

Then he went on the attack against Kamala Harris, the party’s new presumptive nominee, claiming that she was a “radical left lunatic” and the “ultra-liberal driving force behind every single Biden catastrophe”.

Republican groups have been flooding the airwaves in key battleground states, in an attempt to define Ms Harris in their terms, not hers. According to research by the Associated Press, Trump’s side is slated to outspend their Democratic counterparts 25-to-1 over the course of the next month.

One advertisement had been saying Ms Harris was complicit in covering up the president’s “obvious mental decline”.

Mr Biden’s speech offered a nationally televised, primetime opportunity to provide a rebuttal to the attacks against his vice-president and to firmly address concerns about his ability to continue to fulfil his presidential duties.

It was an opportunity he mostly passed on.

Towards the end of his speech, the president did talk up his running mate. He said Ms Harris was “experienced, tough, capable” and an “incredible partner for me and a leader for our country”.

They were strong words, but there weren’t many of them. He spent more time discussing Benjamin Franklin than he did his vice-president – the person he endorsed on Sunday, and the one who will be the most important torch-carrier for his legacy in the months ahead.

With little cover from the president, Ms Harris and her team will have to decide whether, and how, to respond to the withering Republican attacks in the coming days.

Mr Biden may have another chance to tout his former running mate at the Democratic convention in Chicago next month, but this is a delicate time for the new presumptive nominee, as her campaign is just lifting off the ground and Americans are still getting to know her.

The president may have been uncomfortable being overly political in this what could be his final Oval Office address. But if he is concerned about his legacy, Harris’s success or failure, more than anything else he does from here on out, matters.

It will determine whether history judges him as man who made a noble sacrifice, or one who put his party at risk by selfishly holding on to power for too long.