ADEL, Iowa — John F. Kerry was here to talk about Joe Biden. But as he scanned a crowd of about 50 people who had gathered inside a bowling alley in this small town, he couldn’t help but feel a tad nostalgic.

Though it was 16 years ago, his own presidential run suddenly came back vividly.

He had spent time driving down many of these same roads, going town to town during the 2004 campaign. “I fell in love with all your Christmas sweaters. I don’t know what it is about Iowa, but there are more Christmas sweaters here, and they are beautiful,” the former senator and secretary of state said. “I gained a special respect for that and also for measuring my life by the height of your corn.”

It was Iowa that had ignited his bid for the White House, even when many pundits had written him off. In a surprise victory that has been cited repeatedly by underdog candidates like Sen. Cory Booker (D-N.J.), Kerry won the Iowa caucuses that year, defeating Vermont Gov. Howard Dean, a progressive who had led in the polls for months.

That Iowa comeback ultimately helped Kerry claim the nomination, “run around the entire nation and come within one state and half the people in a football stadium of dislodging a president at war,” Kerry said. With a smile, he recalled how the race had even been briefly called in his favor based on faulty exit poll results. “I was president for about five hours on Election Day,” he said. “It was a hell of a scandal-free administration.”

As the former secretary of state scanned the room of smiling, clapping Iowans, his eyes lingered on a man who proudly wore a “Veterans for Kerry” T-shirt from that campaign long ago.

But he wasn’t here to talk about himself or his own ambitions.

“I’m here for Joe,” Kerry said.

For about 20 minutes, the former secretary of state gave a rousing speech on behalf of Biden, his colleague in the Senate for 24 years and in the Obama administration for another four; his longtime friend now in a political fight against a field of candidates that are mostly younger or more progressive.

But with less than a month to go before the Feb. 3 caucuses, the campaign is being waged against a backdrop of escalating tensions with Iran, bringing the question of foreign policy to the forefront in a race that has largely been defined over differences in domestic issues such as health care.

That would seem to give an opening to Biden, who has long campaigned on his extensive foreign policy experience and his ability to walk into the White House on “day one” and rebuild the nation’s relationships with foreign allies that have been strained under the Trump administration.

On Wednesday, Kerry, who was the lead architect of the Iran nuclear deal, argued that President Trump’s actions have increased the nuclear threat and put the world at greater risk.

“No American diplomat, no American citizen is safer in the world today after what he did than they were before. Why else would we be ordering all our people to get out of countries, battening down the hatches, redeploying troops?” Kerry said.

Looking around the room at a group of mostly undecided voters, Kerry said the drama “was a real measure of where we’d be if we had a President Joe Biden.” If Biden were president, “we would still be negotiating the security of the region and the United States using diplomacy as our best tool,” he said. Biden, Kerry argued, would use his “gravitas, experience and relationships” to protect the county and restore the United States’ reputation among foreign allies.

Afterward, Kerry said he believed that aspects of the Iranian nuclear deal were salvageable and that diplomacy was still possible with Iran, but not by Trump. “No Iranian leader would take the risk of sitting down with Donald Trump today. They’d look weak,” Kerry said. But he argued that they would sit down with Biden.

“I mean, it’s going to take work,” Kerry said. “But, yes, the fundamental ingredients are there, because Joe Biden was part of the team that reached out to them and proved our trustworthiness.”

At the microphone, Kerry told the group how “special” Iowa was to the country and to him, how seriously voters there took their responsibility, and the influence they had on the trajectory of the campaign.

“I want you to measure Joe Biden against every single one of the other candidates. They’re all great people. Believe me, I know them. I like them. I respect them. Bright, intelligent, thoughtful, capable,” he said. “But measure their lives against Joe Biden. Measure the accomplishments, because the road ahead is often defined by the road behind, where you’ve traveled, how you’ve traveled. . . . I hope in your minds you will measure the degree to which this moment is critical to all of us.”