Spencer, Iowa (CNN) For days, Bernie Sanders has been eagerly teeing up his policy differences with Joe Biden, drawing clear contrasts with the former vice president in hopes of dislodging him as the early front-runner in the Democratic presidential race.

But near the end of a campaign stop Sunday, after Sanders went on for more than an hour without so much as mentioning Biden's name, Beverly Brix raised her hand and bluntly asked a question.

"I'm on the fence as far as you or Biden," Brix told Sanders. "I like what I heard here today, but how are you different than Biden?"

When the laughter in the room subsided, Sanders started with praise for his rival.

"Joe Biden is a friend of mine, and we worked together on some issues when he was vice president," Sanders said, soft-pedaling his answer as the Democratic crowd listened carefully. "I like Joe."

While Sanders is steadily injecting differences with Biden into the bloodstream of the 2020 presidential campaign -- on trade agreements, the Iraq War and Wall Street policy -- he is hardly screaming it from the rafters.

Sanders made the decision himself to begin drawing distinctions between his and Biden's voting records during television interviews. But when speaking directly to voters here this weekend, he measured his words far more carefully, perhaps mindful of the potential risk of taking on a popular figure in the party like Biden.

It was only after Sanders declared that all candidates must ultimately come together to win the White House, which generated a standing ovation from a crowd of about 300 people, did he finally offer a succinct list of distinctions with Biden. He argued that Biden was on the wrong side of NAFTA, the Iraq war and financial regulations.

"I don't want to go on and on. Joe is a very decent guy," Sanders said. "He's a friend of mine. We have differences of opinion and you all will make your own decisions."

An air of suspicion hangs over Sanders' candidacy among some Democrats, who fear he is pulling the campaign debate too far to the left. That concern is palpable among party officials in Washington and activists paying close attention to the caucus campaign.

While Sanders seldom pulls punches on any other topic, his approach to Biden suggests a hint of awareness on his part that he is still a party outsider trying to avoid being seen as a spoiler in the ultimate fight against President Donald Trump.

"We should all be very clear that we're going to come together to defeat the most dangerous president in the history of this country," Sanders said.

Nine months before Iowa opens the presidential nominating contest, Sanders is not only trying to keep his old supporters on board, who nearly delivered him a victory here over Hillary Clinton in 2016, but he is also working to expand his appeal beyond his loyal followers. He is stepping up his visits to rural Iowa and starting to take questions from voters as most other candidates do.

"The ideas we were talking about were not extreme, were not radical, but were precisely what the American people wanted to see," Sanders said. "Thank you, Iowa."

While many people attending his town hall meeting at the Clay County Fairgrounds Sunday were diehard Sanders supporters, as evident by their faded 2016 campaign shirts bearing his name, several others said they were entirely undecided.

But by confronting Biden's voting record, Sanders has made clear that he sees the former vice president as a threat, even though they represent opposite wings of the party. It's a far different approach than he's taking to Elizabeth Warren, who he has declined to highlight any differences with, even though they overlap more than any other rivals.

"They're friends, they're good people," Sanders said of the large Democratic field, particularly the senators he works closely with. "What we've got to show the world is that we can run a serious, primary process without trying to destroy each other, without character attacks."

As Sanders moved onto his final stop of a two-day Iowa visit on Sunday evening, Brix lingered behind and pondered the exchange.

She said she remained uncertain whether to support Sanders, Biden or another candidate. She also noted how Sanders praised Biden more than she expected, saying: "That's the politician in him, I guess."

Brix said she found Sanders impressive, but he did not resolve a key question in her mind: Does he have broad appeal and can he withstand Trump's attacks?

"I would vote for him if I knew he could win," Brix told CNN. "Beating Trump is No. 1. We have to do that."