President Donald Trump is trying to drag former Vice President Joe Biden's name through the mud ahead of an election year and amid an escalating impeachment inquiry.

Trump in a July 25 phone call urged Ukraine to investigate Biden and his son.

Trump's primary allegation is that Biden squeezed Ukraine into firing its top prosecutor to the benefit of his son, but there's no evidence of this.

Even still, there's growing evidence Trump's efforts are successfully sowing doubts about Biden.

Biden's campaign advisers are worried that Trump is winning the messaging battle, as polling simultaneously shows a significant cohort of voters believe Biden probably pressured Ukraine to fire the prosecutor on his son's behalf.

Visit Business Insider's homepage for more stories.

The Ukraine scandal and related whistleblower complaint pose an existential threat to Donald Trump's presidency.

The revelation that Trump urged a foreign power, in this case Ukraine, to investigate former Vice President Joe Biden has sparked an impeachment inquiry in Congress.

But the controversy also has the potential to sow doubts and suspicion around Biden, despite no evidence of wrongdoing on his part, and there are already signs that it's begun to.

Trump seems to be looking to his 2016 playbook in this regard.

Throughout the 2016 campaign season, Trump sought to malign former Secretary of State Hillary Clinton, his top opponent, by constantly focusing on her use of a private email server.

Biden is a top contender for the 2020 Democratic nomination and an unapologetic critic of Trump, and the president appears to be employing a similar tactic as he seeks to divert attention away from the impeachment inquiry and discredit the former vice president.

And history tells us this strategy just might work.

Ukraine is the new 'but her emails'

Republican presidential nominee Donald Trump (L) and Democratic presidential nominee former Secretary of State Hillary Clinton during the town hall debate at Washington University on October 9, 2016 in St Louis, Missouri. Getty

Though the FBI characterized Clinton's use of a private email server as "extremely careless," it concluded she did not commit any criminal wrongdoing. To this day, Trump has not let the issue go, and even alluded to it in the phone call with Ukraine at the heart of the whistleblower complaint. And he really hammered Clinton on it during the campaign, at one point suggesting he'd jail her if elected.

It's hard to know exactly how much of an influence the focus on Clinton's emails ultimately had in her 2016 loss to Trump, which was a product of a convoluted array of factors and occurred despite her winning the popular vote.

But when Clinton left her post as Secretary of State in 2013, she had a whopping 69% approval rating. Within just a few years she would go on to become the second-most unpopular major-party presidential nominee in modern US history, surpassed only by Trump in this regard. And polling during the 2016 campaign season showed that Clinton's handling of the email scandal was hurting her with voters — including Democrats.

Biden could potentially suffer similar consequences as Trump seeks to make the Ukraine scandal a story about the former vice president rather than his efforts to have a political opponent investigated.

There's no evidence to back up Trump's allegations against Biden, but that might not matter

US President Donald Trump (L) shakes hands with former US President Barack Obama (C) and former vice-President Joe Biden after being sworn in as President on January 20, 2017 at the US Capitol in Washington, DC. MARK RALSTON/AFP/Getty Images

The whistleblower complaint centers around a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky. On the call, Trump urged Ukraine to investigate Biden regarding the former vice president's role in seeing a prosecutor fired at the same time that his son, Hunter Biden, served on the board of a powerful Ukrainian gas company, Burisma Holdings.

Trump's primary allegation is that Biden squeezed Ukraine into firing its top prosecutor to the benefit of his son, but there's no evidence of this.

It's true Biden pressured Ukraine to fire the prosecutor, Viktor Shokin, and as part of these efforts he at one point threatened to withhold $1 billion in loan guarantees from Ukraine if Shokin wasn't pushed out.

But the former vice president was actually among a large number of US and European officials to push for the ouster of Shokin — and the European Union hailed his dismissal — as he was widely viewed as far too soft on rampant corruption in Ukraine. Moreover, by the time Shokin fired there were no active investigations into Burisma.

Long story short, Trump's allegations against Biden are without and evidenced foundation.

Even still, more Americans seemingly believe Biden probably pressured Ukraine to fire Shokin to help his son than those who say he probably didn't, according to a recent Monmouth University poll, which suggests the narrative Trump is pushing is taking hold.

Read more: Trump's stunning call for China to investigate Joe Biden may have given Xi Jinping all the leverage he needs in the trade war

Monmouth found 42% of Americans say Biden "probably did exert this pressure" and 37% say this probably did not happen.

Meanwhile, the new prosecutor general in Ukraine on Friday announced he is reviewing past investigations into the owner of Burisma. This raised the possibility of inquiries into the gas company tied to Biden's son being restarted.

The Ukraine scandal is rapidly escalating, and a second whistleblower with first-hand knowledge of some of the allegations outlined in the complaint has also come forward.

Trump's presidency is in serious trouble. But as this controversy drags Trump down, he appears determined to take Biden with him via an endless stream of attacks on Twitter and in statements to the media. And there are growing signs that if Biden isn't able to mount a potent counteroffensive, Trump might succeed.

'If someone says something enough, people will start to believe it'

Family members gather for a road naming ceremony with U.S. Vice President Joe Biden, centre, his son Hunter Biden, left, and his sister Valerie Biden Owens. AP

Before Biden even officially entered the 2020 race, he was at the top of the polls for months. After he officially announced in April, he continued to lead the pack for weeks and well into summer.

But in the past two weeks, his numbers have begun to dip and other candidates like Sen. Elizabeth Warrenhave pulled ahead of him in multiple state and national polls.

Whether or not this is linked to the Ukraine controversy is difficult to measure, but there is evidence that Trump's baseless allegations against Biden are beginning to shape voter perceptions about the whistleblower complaint and ensuing scandal. And some of his campaign advisers are getting worried.

Democratic Rep. Cedric Richmond of Louisiana, who serves as a Biden 2020 campaign co-chairman, for example, recently told The New York Times, "If someone says something enough, people will start to believe it, and this president gets in his zone of telling a lie over and over again. You have to make sure people don't believe in it."

"It's time to really respond so everybody hears it," Richmond said.