Joe Biden increased his delegate lead over Bernie Sanders on Tuesday night with big wins in the Arizona, Florida and Illinois Democratic primaries, putting the former vice president on course to be the party's presidential nominee.

The scheduled primary in Ohio was postponed as a result of the coronavirus, which has now hit each of the 50 states, just hours before voting was due to begin. Millions of people went to the polls early or voted by mail, and though turnout was up overall in Florida, the Associated Press reported problems with on-the-day voting. 

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The AP Delegate Tracker gives Biden a delegate lead of almost 300 in the race to be the Democratic nominee to take on Donald Trump, who officially became the Republican presumptive nominee on Tuesday after winning the Florida and Illinois primaries, in November's presidential election. 

The 77-year-old Biden has 1,147 delegates, comfortably ahead of Sanders on 861, with allocation ongoing. The Vermont senator is under pressure to drop out of the race, with no clear path to the nomination and several future primaries postponed.  

'I hear you'

Biden gave remarks via live stream from his home in Delaware shortly after the scale of his victories became clear, focusing on the COVID-19 outbreak but also making a pitch to Sanders' supporters. 

The former vice president expressed gratitude to the medics on the front lines of the public health crisis, and sympathy to those affected. "The coronavirus doesn't care if you're Democrat or Republican," he said. 

"It will not discriminate based on national origin, race, gender or zip code. It will touch people in positions of power as well as the most vulnerable people in our society. We're all in this together."

Biden said his campaign had had a "very good night," and praised Sanders for his success in pushing forward issues such as affordable healthcare and action against climate change.  

"Senator Sanders and his supporters have brought a remarkable passion and tenacity to all of these issues. Together they have shifted the fundamental conversation in this country," Biden said. "So let me say, especially to the young voters who have been inspired by Senator Sanders, I hear you. I know what's at stake. I know what we have to do."

Pressure on Sanders

Biden was on course to win over 60 percent of the vote in Florida, which allocates 219 delegates, with Sanders trailing on around 22 percent when nearly all of the votes were counted.

The former vice president also held a 20-plus point lead in Illinois and a double-digit advantage in Arizona with final results still coming in.  

Sanders was set for an even bigger loss in Florida than the 31 points he was defeated by in 2016, when Hillary Clinton won a comfortable victory in a state which Trump later took in the general election. It is set to be a pivotal state once again in the 2020 presidential election. 

CNN polling suggested Biden won overwhelmingly in Florida with people over the age of 65, moderates and black voters. 

White male voters in the Sunshine State also moved decisively away from Sanders having backed him in 2016, and the Vermont senator must now decide if he should stay in the race to make the case for his policies despite not having a realistic path to victory.