BURLINGTON, Iowa—For a politician, Joe Biden is fairly polite. All the same, the former vice president fancies himself a brawler. Hence his old daydream about getting into a fist fight with the current president of the United States.

If the elder statesman were a younger man, a much younger man, he wouldn’t have let Donald Trump get away with the kind of sexist vulgarity heard on the 2016 “Access Hollywood” video. How would the 77-year-old Democrat set the 73-year-old Republican straight? “If we were in high school,” Biden told University of Miami College Democrats ahead of the 2018 midterms, “I'd take him behind the gym and beat the hell out of him.”

It was blue collar and old school and scrappy, all the things that endear the candidate to a primary season electorate desperate for a champion to knock out Trump, and though Biden later apologized for his imaginary bellicosity, he has not let up on the president.

Biden has, however, avoided anything remotely resembling aggression in the primary. He attacks the president. He lays off primary opponents. How much longer can a genuinely nice guy play nice without risking his shot at the nomination? It is a question the Biden campaign will have to answer if the candidate fails to live up to early expectations. It is not, for now, a question that seems to have crossed his mind.

Biden hosted three events three days before the first-in-the-nation caucuses. He lambasted Trump repeatedly at each one. He never said a critical thing about the other Democrats clamoring to knock him from his perch, including Bernie Sanders, who is surging in Iowa, New Hampshire and California. According to the RealClearPolitics polling average, the democratic socialist from Vermont leads in all three, and according to a new NBC News/WSJ poll, Sanders is in a virtual tie with Biden nationally.

Those numbers have some Democrats begging the old bull to take the gloves off before it is too late.

“They should go after Bernie. People just don’t know the depth of what’s out there,” said Matt Bennett, a founder of the centrist Democratic group, Third Way, referring to Sanders’ long and somewhat controversial history. “That’s why we are doing our thing, but only candidates can break through.”

Some centrist Democrats fear the fate that befell the Republican establishment in 2016, when Trump steamrolled an entire field of moderate GOP candidates. Many worry that just like Trump, Sanders could gain so much momentum that he would become impervious to challengers and run away with the nomination.

No candidate on the left has opened fire on Sanders for his longtime embrace of socialism, demonstrating a kind of restraint that Trump never has. The Republican National Committee is already preparing to make the general election a referendum on socialism. If Sanders were to become the nominee, the GOP would gleefully attack him for his “honeymoon” in the Soviet Union, his more radical writings, and his praise of the late Cuban dictator Fidel Castro.

Biden has avoided such temptation. The candidate has instead run his campaign as if he were already the nominee, focusing his ire on Trump and pitching himself as the very antithesis of that brash president. He dubbed his latest trek through Iowa as the “Soul of the Nation” bus tour, harkening back to his promise to promote unity rather than division.

“If the American people want a president to add to our division, lead with a clenched fist, closed hand, a hard heart, to demonize the opponents and spew hatred — they don’t need me,” Biden said in Philadelphia when he kicked off his campaign last May. “They’ve got President Donald Trump.”

Biden has not turned the other cheek though. If the former vice president won’t throw the first punch, he has not hesitated to push back. When former South Bend, Ind., Mayor Pete Buttigieg recently accused the frontrunner of reading from an old “Washington playbook” and “recycling the same arguments,” for instance, Biden responded by telling reporters that he has “gotten more than 8,600 votes in my life.” The dig was a reference to Buttigieg’s margin of victory in his second term as mayor of South Bend, an office some rivals say was too minor to prepare him for the world stage.

But other than rebuttals, Biden has not hit first. His campaign clearly does not feel the need. Attackers, most notably including California Sen. Kamala Harris, who challenged him on the debate stage, have become also-rans. And when a Sanders campaign surrogate accused the frontrunner of being “corrupt” in an op-ed published in The Guardian last month, Sanders himself issued a public apology.

This has left Biden mostly unbloodied. What incentive does his campaign have to start a fist fight? asks John Zogby. “He is a former vice president, so it is a matter of stature and age. Sharp elbows could be seen as unbecoming,” the longtime Democratic pollster told RCP. “Besides, he will need everyone’s support if he’s nominated.”

At least two Biden supporters seemed content with the restraint.

“If you have enough confidence in your own abilities, you just don’t need to go after other people,” said Ann Metzger, a retiree from Des Moines. She was a caucus leader for Sanders in the last primary but has switched allegiances for the current election because she believes he is the best candidate to defeat Trump. “Joe has so much class that he would do well to just stay the course.”

Anything else, insisted Bruce Dunele, would be out of character. “He is tough as nails. He is a man who can be tough as iron and also as soft as a pillow,” the retired electrical engineer from Burlington said as he grabbed two Biden yard signs.