DES MOINES, Iowa — As Joe Biden barnstorms Iowa before Monday's all-important caucuses, his rivals for the Democratic presidential nomination are cautiously flagging a high-intensity battery of Republican attacks on him as warning signs for the party.

Andrew Yang suggested Wednesday that it was wrong for Biden's son Hunter to sit on the board of a foreign company when Biden was vice president.

"My administration’s going to be about trying to establish a tone and a culture in D.C., and I don’t think my eventual VP would head down that road," Yang said at a Bloomberg News breakfast roundtable with reporters here.

That kind of subtle contrast is about as far as any of Biden's Democratic rivals will go toward publicly criticizing him, because they see little benefit to their own campaigns — and tremendous risk — in piling on at a time when Republicans are barraging voters here with allegations that members of Biden's family benefited improperly from his positions in elected office.

That's especially true given the fact that Trump himself has amplified unsubstantiated charges against Biden.

But at the same time, when speaking on the condition of anonymity, aides to some of the other candidates use the GOP's monthslong bombardment of Biden to warn that President Donald Trump could turn his own Ukraine scandal into an effective tool against the former vice president.

"This is a Clinton 'emails' scenario all over again," said one operative who spoke to NBC News. "Right now, Democrats are focused on electability and beating Trump while we're on a collision course to have a total repeat of 2016 if we nominate Biden. It's Hillary 2.0."

Democratic voters wrestling with conflicting instincts — whether to frustrate Trump by rallying to Biden's side, or to run from Biden out of fear that he's wounded — won't get much help from the public statements of the other Democratic candidates. They are mostly taking the safe route of offering mild defenses of Biden, typically saying that he is a "distraction" from Trump's troubles, while letting the GOP do the handiwork of roughing him up.

Trump's impeachment response, mounted in the home stretch of the Iowa race, has centered around the idea that he pursued an investigation into Biden because of concerns about corruption, rather than to help his own re-election campaign.

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Both House managers and Trump's lawyers have spoken at length about Biden and his son Hunter Biden, who was hired to sit on the board of Ukrainian natural gas company Burisma despite having no relevant experience. Trump and his allies have promoted an unfounded theory that Joe Biden's effort, along with that of several European countries, to force the firing of a Ukrainian prosecutor several years ago was aimed at helping Burisma.

The United States was working in conjunction with European partners to seek the prosecutor's dismissal over concerns he was failing to aggressively fight corruption; the probe into Burisma had long been inactive at the time; and there has been no evidence that Biden acted improperly to benefit his son.

The discussion of the Bidens on the Senate floor provided new fodder for GOP lawmakers to lob volleys at the national front-runner for the Democratic presidential nomination in the closing days before the nation's first contest.

"Iowa caucuses are this next Monday evening,” Sen. Joni Ernst, R-Iowa, said this week. “And I’m really interested to see how this discussion today informs and influences the Iowa caucus voters, those Democratic caucusgoers. Will they be supporting Vice President Biden at this point?"

Sen. Rick Scott, R-Fla., made a small buy for an ad here attacking Biden on Ukraine that included the same basic unfounded allegation Trump has promoted.

Those are obvious signs, Biden said, that Republicans fear him more than they do any other prospective Trump opponent.

"They’re smearing me to try to stop me because they know I’ll beat Donald Trump like a drum," he said Wednesday at a campaign stop in Sioux City, Iowa. The argument echoes what his camp has said about the extraordinary effort Trump undertook to have him investigated by a foreign power.

The former vice president says that he's shown resilience in the face of being a nonstop talking point for Trump since the Ukraine scandal broke in September.

"I understand this new Republican Party better than anybody — I've been the object of their affection for a while now," he said Tuesday in Iowa City. "They have thrown everything but the kitchen sink at me so far and I'm going to just say it, I'm still standing and I'm getting stronger."

Polling bears him out, at least on the first part: Biden was at 20.3 percent in the Real Clear Politics average of Iowa polls on the day Trump released a White House summary of his phone call with Ukrainian President Volodymyr Zelinskiy in September and at 21 percent on Wednesday. On the national level, Biden was in first place at 29 percent on Sept. 25 and still in the lead at 28.1 percent on Wednesday.

Joe Galasso, 62, of Waukee, said the Ukraine affair plays "zero" role in his thinking as he weighs whether to caucus for Biden or Sen. Amy Klobuchar, D-Minn.

"Should his son have taken that job? Probably not," Galasso said, before moving on to the question of why Biden pushed for the firing of the Ukrainian prosecutor. "Do I think Joe did that for his son? No."

He added that he's not worried about Trump smearing Biden later on because he knows it will happen.

"Trump is going to lie no matter what," he said.

Yang declined to say whether he thought Biden has been effective in combating Republican barbs over Ukraine. Instead, he focused on Trump.

"I personally see trying to drag Joe into this as like just a political distraction," he said. "I mean, the president's actions were inappropriate, regardless of anything that was meant to be probed or investigated. ... I don't think most people see that as core to the inquiry."

Asked a second time about the effectiveness of Biden's response, Yang added another point of comparison, rather than addressing Biden's adeptness or dismissing the attacks on him out of hand.

"I just think that for this administration to try and paint Joe or his family as corrupt just seems sort of ridiculous," he said.