DES MOINES — With President Trumpholding a campaign rally here Thursday ahead of the Iowa caucuses, his allies had coalesced around an effort to damage former vice president Joe Biden's prospects in the state that begins the 2020 presidential nominating contests.

Republican lawmakers have used the spotlight of Trump’s impeachment trial to accuse Biden of corruption, a U.S. senator from Florida is running television ads against Biden in Iowa, and several Trump campaign surrogates have ramped up their attacks against the former vice president as they prepare to fan out across the state in support of Trump’s reelection bid.

“There is a mountain of evidence to suggest the Bidens’ behavior was harmful to the United States,” Sen. Lindsey O. Graham (R-S.C.) said Wednesday from the U.S. Capitol, where several GOP lawmakers used the impeachment trial’s question-and-answer session to insinuate wrongdoing by Biden.

Trump’s campaign is dispatching Vice President Pence and more than 80 other surrogates to Iowa ahead of Monday’s caucus votes. The group includes several Republicans who have launched the most pointed attacks against Biden and his son Hunter during the impeachment process.

Trump’s rally in Des Moines came on the eve of a critical Senate vote over whether his impeachment trial would include witnesses. The president has used his recent rallies to criticize several of his Democratic rivals, with Biden receiving the lion’s share of the attacks.

Biden responded Thursday by drawing a sharp contrast with Trump in a speech and with a new television ad, both aimed at highlighting differences in character between the two men and previewing a potential general-election matchup.

“In Joe Biden’s America, the president’s tax returns will not be a secret,” Biden told a standing-room-only crowd at an elementary school in Waukee, Iowa, his first of three stops for the day. “Political self-interest will not be confused with national interest. And no one, not even the president of the United States, will be above the law.”

“Character is on the ballot. America’s character,” Biden’s prepared remarks said. “I do not believe we’re the dark, angry nation we see in Donald Trump’s tweets.”

On Thursday, Trump mocked several of his Democratic rivals during a speech that stretched longer than an hour, declaring Biden’s campaign “over.”

“Joe had a crowd so small the other day that they set up a round table,” Trump said.

Earlier in the rally, he decried the impeachment process as a “deranged, witch-hunt, hoax“ driven by partisan “rage.”

After former Florida attorney general Pam Bondi, a member of Trump’s impeachment defense team, spent 30 minutes Monday echoing the president’s attacks on Biden from the Senate floor, Sen. Joni Ernst (R-Iowa) bluntly assessed the political impact the broadside could have on the first-in-the-nation caucuses.

“Iowa caucuses, folks, Iowa caucuses are this next Monday evening,” Ernst told reporters on Monday. “And I’m really interested to see how this discussion today informs and influences the Iowa caucus voters, those Democratic caucus-goers. Will they be supporting Vice President Biden at this point? Not certain about that.”

Biden, who has denied any wrongdoing, has responded by arguing that Republicans’ focus on him signals their fear that he is the strongest candidate to defeat Trump in November.

“Joni Ernst just spilled the beans,” Biden wrote on Twitter. “She and Donald Trump are scared to death I’ll be the nominee. On February 3rd, let’s make their day.”

Biden’s campaign has also pushed back against the onslaught, criticizing Republican threats to call Biden and his son as witnesses in Trump’s impeachment trial as partisan attempts to interfere in the 2020 race.

Trump stands accused of withholding military aid and a critical White House meeting from Ukraine to pressure the Eastern European nation to announce investigations into the Bidens. In her presentation at Trump’s trial, Bondi accused Biden of corruptly pressuring Ukraine to fire a prosecutor in 2015 while his son Hunter served on the board of a Ukrainian natural gas company.

Biden campaign officials pointed out Thursday that the Republican allegations against Biden are unfounded or have been debunked.

“Trump desperately wants to impact the outcome of the Democratic primary, dropping into Iowa a few days before the caucus to spread a message of division, discord, and hate,” Biden’s campaign said in a statement previewing Thursday’s speech. “Trump has been trying to prevent Biden from getting the nomination since the moment the VP got into the race, getting himself impeached by the House and tried in the Senate in the process.”

Biden’s campaign also released the new ad, which runs for one minute and is titled “Character.” It criticizes Trump’s behavior in the White House while describing Biden as a steady leader. “It’s said, ‘In here is where your character is revealed,’ ” the ad says, flashing an image of the Resolute Desk in the Oval Office. “But it’s in life where your character is formed.”

The ad ran Thursday in Iowa’s top five media markets, Biden’s campaign said.

Although Biden events can resemble classroom lectures on electoral math, the former vice president is wrapping up his Iowa bid by talking less about policy and more about character. Trump, he said Thursday, has led a “culture of cruelty” from the White House.

“This is a president who laughs at, insults and demeans other people. He pits us against one another. He traffics in the ugliest lies,” Biden said in Waukee. “He’s more bully than president. He’s more George Wallace than he is George Washington.”

Several polls have shown Biden leading Trump nationally and in key swing states in a hypothetical general-election matchup.

Polling in Iowa shows a close race in the Democratic caucuses, with Biden, Sen. Bernie Sanders (I-Vt.), Sen. Elizabeth Warren (D-Mass.) and former South Bend, Ind., mayor Pete Buttigieg all vying for a victory on Monday.

Trump does not face serious competition in the Republican caucuses, which also take place Monday.

His campaign’s decision to send more than 80 surrogates into the state is aimed at engaging Trump supporters rather than affecting the outcome of the close Democratic race, said Kayleigh McEnany, a Trump campaign spokeswoman.

“We’re not there to shape the Democrat outcome,” she said. “We’re there to showcase the support for the president.”

Still, many of the surrogates will inevitably highlight the “contrast” between Trump and some of the leading Democrats, she said.

During a “Women for Trump” event this month in Des Moines, McEnany and Lara Trump, the president’s daughter-in-law, criticized Biden and Sanders as too liberal for the country.

Lara Trump is listed as one of the dozens of surrogates the Trump campaign plans to send to various caucus sites to meet with supporters and engage with local media in Iowa. Others include Donald Trump Jr., Eric Trump, Iowa Gov. Kim Reynolds (R), acting White House chief of staff Mick Mulvaney and several Cabinet secretaries.

Several Republican members of Congress who have been leading defenders of Trump against impeachment will also participate. Among them will be Reps. Jim Jordan (Ohio), Mark Meadows (N.C.), Matt Gaetz (Fla.) and Elise Stefanik (N.Y.)

“Our Caucus Day operation is just a preview of what is to come,” Brad Parscale, Trump’s campaign manager, said in a statement this week. “This will be the strongest, best funded, and most organized presidential campaign in history. We are putting the Democrats on notice — good luck trying to keep up with this formidable reelection machine.”

Some non-surrogates are also seeking to influence the Iowa race. Sen. Rick Scott (R-Fla.) began running television ads this week in Iowa accusing Biden of “corruption.”

The concerted attacks on Biden could end up “rebounding against” Trump and his allies, said Jim Messina, who was campaign manager for President Barack Obama’s 2012 reelection bid.

“Donald Trump’s focusing on Joe Biden has helped Joe Biden during this primary process,” he said. “Democrats understand that Trump is worried about Biden. And it helps Biden’s electability argument, that he’s the candidate who could best beat Donald Trump.”

David Weigel in Council Bluffs, Iowa, and Cleve R. Wootson Jr. in Waukee, Iowa, contributed to this report.