Washington (CNN) He's the Democratic candidate President Donald Trump has long worried would emerge as his general election rival. Now, with the entry of former Vice President Joe Biden into a crowded primary field , a race that once seemed to Trump like abstract chatter has sharpened into a concrete proving ground that will challenge his hold on the nation's political spotlight.

Already, Trump has noticed with unpleasant surprise that the Democratic field is occupying more and more airtime and print inches -- space he has enjoyed largely to himself for the past two years. He's worked to counter the tide, returning to his practice of conducting phone interviews with friendly television hosts and increasing his angry output on Twitter.

But with Biden's official entry into the Democratic field , Trump will now contend with a front-runner whose opening argument went directly at his fitness for office, and whose strategy will include peeling away the lower-income white voters who propelled Trump to the White House.

It did not take long for the President's official response.

"Welcome to the race Sleepy Joe," Trump tweeted less than three hours after Biden launched his campaign with a direct-to-camera video assailing the President for his response to white supremacist violence in Charlottesville, Virginia

"I only hope you have the intelligence, long in doubt, to wage a successful primary campaign," Trump wrote. "It will be nasty - you will be dealing with people who truly have some very sick & demented ideas. But if you make it, I will see you at the Starting Gate!"

Asked outside a pizza shop in Wilmington, Delaware, for his response, the freshly declared candidate did not enter the fray.

"Everybody knows Donald Trump," Biden said before climbing into the front seat of a black sedan.

All about authenticity

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As the Democratic primary enters a new phase, Trump has increasingly tuned into the race as candidates begin regular appearances on cable news and on the campaign trail.

Both in their seventies and each with a political brand rooted in authenticity, Trump and Biden -- were they to face-off -- would compete for voters in the Rust Belt and the Midwest, where Trump won traditionally Democratic enclaves by appealing to a sense of economic and cultural unease.

At the same time, Biden represents a vestige of the Obama administration, which Trump has fixated upon as he compares his own presidency to that of his predecessor.

In political meetings at the White House, Trump often asks about Biden's strength, particularly as his candidacy has grown more certain in recent weeks. Biden's native Pennsylvania -- Trump's favorite state that he switched from blue to red in 2016 -- is at the root of the President's concern. Biden's first rally will be in Pittsburgh on Monday

"If he would make it to the general election, yes, he's a problem," a Republican involved in the meeting said, speaking on condition of anonymity to discuss a private meeting. "But how can he survive a Democratic primary?"

That, of course, is a central question that won't be answered for months. But Republicans are taking Biden as seriously as any candidate -- and more seriously than most.

"I still think of all the 20 candidates currently in the race, the former vice president speaks directly to those 90,000 swing voters in Pennsylvania, Wisconsin and Michigan that made Donald Trump president," said David Urban, a senior adviser to the Trump campaign in Pennsylvania. "If voters wanted a third Obama term, they would have voted for Hillary (Clinton) over Donald Trump."

Once assured to factor at the top of news broadcasts and on newspaper front pages -- and still himself a major news story -- Trump has taken notice as Democrats begin consuming more of the political oxygen.

That includes on his favorite channel Fox News, where more candidates are appearing in prime-time town halls, sometimes to Trump's irritation. He recently griped about Sen. Bernie Sanders' appearance, claiming the network had rigged a town hall audience in Sanders' favor and complaining about the network's moderators.

According to people around him, those complaints reflect a private irritation that he no longer has the airwaves essentially to himself, a status he exploited in the first years of his presidency by swamping cable news broadcasts with tweets and impromptu appearances before cameras.

Now, networks (including Fox) no longer air all of his appearances live, and more attention is being paid to the Democratic field as it expands to include 20 competitors. For more than half-an-hour Thursday, cameras were trained through the window of Gianni's Pizza in Wilmington as Biden made his first public appearance as an officially declared candidate. When he finally did emerge, he mostly declined the opportunity to answer questions -- a stark contrast with Trump, who rarely walks past a bank of cameras without stopping.

Biden on Trump's mind

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In response, the President is planning his own direct appeals to the conservative base of voters unlikely to be drawn away by Democrats. He's phoning into Sean Hannity's Fox News program on Thursday, will speak to the National Rifle Association at their annual conference in Indiana on Friday, and will appear at an evening campaign rally in Wisconsin on Saturday.

Biden's entry was long expected, and Trump has been polling his advisers for months on the former vice president's chances. People familiar with the conversations say Biden has factored into Trump's 2020 thinking more than any other candidate, though the President has made private assessments about several of the Democrats as each of them enters the race or enjoys a moment of national attention.

He has denied to reporters that he views the former vice president as a danger, telling CBS News in an interview last year he "dreamed" of running against Biden. Earlier this month he insisted Biden was saddled with a long and ignominious record -- including during the Obama administration, which Trump has blamed with increased frequency for foreign and economic policy blunders.

"I don't see Joe Biden as a threat. No, I don't see him as a threat. I think he is only a threat to himself," Trump said. "He's been there a long time. His record's not good. He'd have to run on the Obama failed record."

And he's worked to drive a wedge between Biden and other Democrats, claiming the swirl of inappropriate behavior allegations that arose earlier this month was the work of Biden's left-leaning rivals.

"It looks like the only non, sort of, heavy socialist is being taken care of pretty well by the socialists, they got to him, our former vice president," he said at a Republican fundraiser. "I was going to call him, I don't know him well, I was going to say 'Welcome to the world Joe, you having a good time?' "