"With the utmost respect for Bernie Sanders, who is an incredible human being & a genuine inspiration, I don't endorse Joe Biden," Gray tweeted Monday.
"President Trump's supporters will run through a brick wall to vote for him. Nobody is running through a brick wall for Joe Biden," the president's campaign manager said.
Trump narrowly beat Hillary Clinton in several battleground states that Obama had won in 2008 and 2012. Democrats aim to win back these states in 2020.
In his first major interview since suspending his presidential campaign, Vermont Senator Bernie Sanders said he hoped to work with former Vice President Joe Biden and "move him in a more progressive direction."
Did Sanders' campaign fail or succeed? Expanded health care, a higher minimum wage, free college: His ideas may have outgrown the need for Sanders himself and his purist approach.
Sanders said on Wednesday that Joe Biden is "going to have to bring new people into his political world" in order to win over Sanders' supporters.
"Joe Biden has the humanity, empathy and steadiness we need in a leader," The Lincoln Project said Wednesday.
"Sanders never wanted to win, like so many ideologues, he wanted to lose. It makes him feel virtuous," the Fox News host said.
"The COVID-19 epidemic highlights the folly of tying health coverage to jobs," a co-author of the analysis told Newsweek.
Recent poll results show Biden holding leads over Trump in both national and statewide polling in key states.
Democratic voters across the country largely rejected the candidates pushing for universal health care in the primary race.
Without Senator Bernie Sanders, the 2020 presidential election will be a race between former Vice President Joe Biden and President Donald Trump.
The delegate math eventually became stacked against Sanders in 2016, and that is now the case for him in 2020.
"Today I am suspending my campaign. But while the campaign ends, the struggle for justice continues on," Sanders tweeted.
"We are listening to our people and, you know, we've got to make the best decision that we can," the progressive senator from Vermont said.
The majority of Florida voters also said they disapprove of Trump's handling of the coronavirus pandemic.
The comedian and Bernie Sanders doppelgänger said that the Vermont senator "should drop out" of the 2020 presidential race, telling a New York Times columnist over the weekend that "everybody" should support former Vice President Joe Biden's campaign.
The Democratic presidential candidate commented that it's easier for status quo forces to maintain a broken system than to fix it.
"We urge you to fully commit to eliminating all patient cost-sharing for COVID-19," the groups wrote in an open letter.
Progressive lawmakers, including presidential candidate Bernie Sanders, have argued that the crisis shows a universal health care system is needed.
"What we're not talking about right now, Seth, is that if you're making 14 bucks an hour, 12 bucks an hour and that check stops coming in, which is the case, how are you going to feed your family? How are you going to pay the rent?" Bernie Sanders said during a Monday appearance.
A poll by Harvard and Harris conducted from March 24 to 26 shows Biden beating Trump by 10 points, 55 percent to 45 percent.
Nearly twice as many supporters of the president said they are "very enthusiastic" about their candidate, compared to those backing the former vice president.
Democratic presidential candidates Joe Biden and Bernie Sanders joined state lawmakers in New York and California in calls to freeze and forgive rent payments, as millions of Americans struggle to make ends meet under coronavirus lockdown.
Most every Democrat and progressive will support Biden, regardless of who is selected #2. But the real Democratic imperative is to win back the large number of blue collar voters who abandoned Hillary in 2016.
Although the New York governor is not running for the Democratic nomination, more people are putting their odds on him over the senator from Vermont.
The Vermont senator mockingly said the universe was collapsing because American workers would get an extra $600 a week on unemployment payments.
Bernie Sanders' campaign said earlier this week that the senator would take on the former vice president again if a debate is held in April.