"I believe everything the president's said and done encourages white supremacists," Biden said as Trump continued to take criticism for his handling of back-to-back mass shootings. One of the shooters is believed to have written a racist screed echoing some of Trump's incendiary language about immigrants.
Bullock said Trump's rhetoric and behavior — attacking his critics on the day he traveled to ostensibly console victims' families in El Paso, Texas, and Dayton, Ohio — are beneath the office he holds.
"The lies and the statements that divide us by race, gender, geography — we expect more out of our preschoolers now than we do the president of the United States," Bullock said.
Biden and Bullock are the first of more than 20 candidates — nearly all Democrats — who will speak at the Iowa fairgrounds over the coming days. It's a rite of passage for would-be presidents, and for Democrats it highlights their challenges in a state like Iowa, a presidential battleground that mixes cultural conservatives and farm country, liberal college towns and plenty of swing voters in between.
Trump won Iowa comfortably in 2016 after President Barack Obama, with Biden as his running mate, won the state twice. The typical Democratic caucus electorate, meanwhile, leans far more liberal than the general electorate.
Biden and Bullock promised to compete both for caucus votes and for the state in November.
"If we can't win back places we lost, ... if we can't give people a reason to vote for us and not just against him, Donald Trump will win again," Bullock said. "The path to victory doesn't just go through the coasts."
But first, the caucus is critical for both men — for different reasons.
Biden, as the national polling leader since he entered the race, needs to demonstrate the strength of a front-runner. Bullock, meanwhile, is a longshot looking for a slow, steady rise from a state where he has family connections and is comfortable selling his Montana record.
Bullock faces an uphill battle in part because of Biden's standing: Both men hail from the more moderate-to-liberal core of the party, warning that a progressive lurch could help re-elect Trump. But the 53-year-old governor is dwarfed by the universal name recognition of the 76-year-old former vice president.
The two candidates Thursday reaffirmed their support for new gun restrictions — expanded background checks, bans on certain military-style weapons and ammunition — even as they stopped short of some solutions their more liberal opponents have proposed.
Bullock said he opposed mandatory buyback programs for military-style weapons, even if Congress bans future manufacture and sales of those guns. Biden said he's wary about using executive orders to do anything other than tinker with background checks. He said banning weapons and ammunition must be done through Congress.
California Sen. Kamala Harris has said she'd use sweeping executive action on guns if Congress doesn't act within 100 days of her inauguration.
Biden and Bullock dismissed the notion that their positions on guns could be at odds with their promises of winning back areas of the country that flipped to Trump in 2016.
Bullock noted that he's vetoed some measures backed by gun lobby, yet won re-election in Montana anyway on the same day Trump won the state by 20 percentage points.
Biden, in turn, quickly dismissed some arguments used to push back on restricting some guns and ammunition.
"It violates no one's Second Amendment rights to say that you can't own certain weapons," Biden said, a reference to the Supreme Court stating explicitly that some regulations are permissible.
Off the fairgrounds, candidates grappled with how far to go in critiquing Trump. Since the El Paso shooting, Texas Rep. Beto O'Rourke has called Trump a white supremacist. South Bend, Indiana, Mayor Pete Buttigieg agreed with that label when asked at a forum in Miami sponsored by the National Association of Black Journalists.
But campaigning in Sioux City, Iowa, Harris stopped short of calling Trump a white supremacist.
"I think that there's no question that his words and his language ... (have) been about condoning the conduct and certainly accommodating the conduct of white supremacists," the California senator told reporters. "So, I think it's a fair conversation to have."
Pressed on the question, Harris, one of two major black Democratic candidates, said, "I think you should ask him that question."
Biden accused Trump on Wednesday of "fanning the flames of white supremacy."
Associated Press writer Thomas Beaumont contributed from Sioux City, Iowa and Julie Walker contributed from Miami.
Follow Barrow on Twitter at https://twitter.com/BillBarrowAP .