John F Kennedy and Ronald Reagan had their speeches in Berlin. Joe Biden now has Kyiv, a moment to define his presidency and its era.

There was no one phrase in Biden’s remarks in Kyiv to match Kennedy’s “Ich bin ein Berliner” in 1963 or Reagan’s “Mr Gorbachev, tear down this wall” in 1987, but the trip itself was the statement. As the White House underlined repeatedly on Monday, there was no precedent in modern times. Visits to the troops in Afghanistan and Iraq were different, as the US military ran security in those countries.

In going to Kyiv, Biden was entering a war zone and putting his safety in the hands of the Ukrainian armed forces, and also those of the Russians. Moscow was given a heads-up a few hours before he crossed the border. The calculation was that Vladimir Putin would not risk the precedent of presidential assassination or all-out war for that matter. A reasonable calculation but a risk nonetheless.

It was a coup heightened by complete surprise. The secret did not leak, signalling that the bravery was underpinned by competence. The visit cemented Biden’s claim of leadership of the free world, but among Washington’s allies that has not really been challenged since the full invasion of Ukraine began a year ago this week.

A tougher question to answer – and it may take a week or two before the result is clear – is whether this will help Biden’s standing at home, where his popularity has not recovered from the hit it suffered from the shambolic Afghanistan withdrawal, inflation and the energy price shock of the invasion.

The popularity slump, which began in August 2021, has not so far been reversed by recent strong economic figures, a solid legislative record, and a lively, combative performance in his State of the Union address earlier this month.

In an average of recent polls, Americans who disapprove of his performance outnumber those who approve by 52% to 42%.

Much of the problem is an overall impression that Biden at 80 is too old, too doddery and gaffe prone to lead the country with vigour, especially into a second term. The bold appearance in Kyiv, strolling through the city in aviator sunglasses, alongside a grateful and admiring Volodymyr Zelenskiy, on the US Presidents Day holiday no less, is intended to address that perception head on and reframe the conversation on age and fitness for office.

Donald Trump was notably risk averse as president. On his single visits to Afghanistan and Iraq, he stayed inside heavily fortified US bases. The Kyiv visit, with its very real jeopardy, makes it less likely that Biden’s Republican challenger in 2024, whether it is Trump or the Florida governor, Ron DeSantis, or another, will challenge him directly on courage. But the Republicans are already pivoting to portraying the president’s starring role abroad as an abandonment of suffering Americans at home.

“I and many Americans are thinking to ourselves: OK, he’s very concerned about those borders halfway around the world. He’s not done anything to secure our own borders here … we have a lot of problems accumulating here,” DeSantis told Fox TV.

The very success of the Biden visit in underlining the US’s commitment to Ukrainian resistance could end up accelerating the drift of the Republicans towards anti-Ukrainian positions, now the preserve of a pro-Trump minority on the far right of the party, as the leadership looks for attack lines against Biden.

In his Fox interview, DeSantis downplayed the Russian threat. “I think it’s important to point out, the fear of Russia going into Nato countries and all that, and steamrolling, that has not even come close to happening,” he said, sketching out what may become the Republican line in 2024.

The conventional wisdom, reinforced by decades of polling, is that foreign policy does not tend to sway presidential elections. What Kennedy and Reagan’s famous Berlin speeches would have done for them electorally is unknown. Kennedy was killed before he could stand for a second term, and Reagan had already been re-elected and was in his penultimate year in office.

For Biden, the jury is out. The train ride to Kyiv will go down in history, but making history does not necessarily win elections.