By Steve Holland
WASHINGTON (Reuters) - Joe Biden's rapid emergence as front-runner in the race for the 2020 Democratic presidential nomination has caught the attention of President Donald Trump and unnerved some of his allies, who believe Biden is a potentially dangerous rival.
Biden soared from "will-he-or-won't-he-run?" status to the head of the crowded Democratic field just days after announcing his candidacy last week, pulling away from Senator Bernie Sanders and a host of other rivals in opinion polls.
A CNN poll gave Biden a 15-point lead in a field of 20 Democratic candidates.
Some Trump advisers see the former vice president, with his mainstream blue-collar appeal, as a tough opponent in the three states that carried Trump to his improbable victory in 2016 - Michigan, Wisconsin and Pennsylvania.
Losing any of those states in 2020 would make Trump's path to re-election more problematic.
"They think they're in trouble there and they think he's a real threat," said one outside Trump adviser, who spoke on condition of anonymity.
Trump's initial response to Biden's entry into the race was to use a well-worn playbook - give him a mocking nickname - "Sleepy Joe Biden" - and insult his intelligence.
"I have known Joe over the years," Trump said on Fox News. "He is not the brightest light bulb in the group, I don't think, but he has a name that they know."
Trump later went on a Twitter tirade after the largest U.S. firefighters union endorsed Biden, including posting dozens of retweets on Wednesday from purported firefighters and their friends and families professing support for the Republican president or criticizing Biden.
Despite his focus on Biden, some of Trump's friends said he did not seem particularly concerned about him, at least at this stage, given that the first votes in the nominating process will not be cast until early next year.
White House counselor Kellyanne Conway played down the idea that Trump saw Biden as his biggest threat.
"No, I think it's just fun to remind everybody about him. ... Maybe he's an easy mark, and he just announced for president of the United States," she told reporters.
Trump allies said Biden, who was vice president under Trump's predecessor, Barack Obama, could still get chewed up by a large, diverse field of Democratic rivals, many leaning to the left of Biden and discussing policies such as tax hikes for wealthier Americans and government-run healthcare.
As long as Democrats are moving left, said David Urban, a political consultant and Trump campaign aide in 2016, "I like our chances whoever is at the top of the ticket for the Democrats."
The Trump re-election campaign said it believed that whoever emerges as the Democratic nominee would have adopted policy stances out of tune with most Americans.
"There is no centrist lane in the Democrat primary," said Trump campaign communications director Tim Murtaugh. "We view them as one big liberal organism right now with 28 heads."
Still, there are warning signs for Trump.
He won narrow victories in 2016 over Democrat Hillary Clinton in Michigan, Wisconsin and Pennsylvania with the help of blue-collar voters who once voted Democratic but switched to Trump. Trump's advisers said repeating his victories in those crucial states next year might be a tall order.
Biden, a longtime U.S. senator from Delaware, held his first campaign rally in Pennsylvania and has quickly sought to make the campaign into a battle against Trump rather than against his rivals for the Democratic nomination.
One Trump confidant said poll numbers indicated Biden stood to do well at this point at least in Pennsylvania and Wisconsin and potentially in two other Trump states, Florida and North Carolina.
If that were to happen, the confidant said, it would be hard for Trump to make up the losses with wins in other states in order to get to the 270 Electoral College votes needed for victory.
"There’s not really a lot of room to expand the (electoral) map over the previous results," the confidant said.
In a response to Biden, Trump has touted strong economic indicators in Pennsylvania, where unemployment is at a historic low.
But some allies said Trump had not been disciplined about selling his record on the economy and that his lack of a healthcare overhaul plan was hurting him along with his preoccupation with migrants at the U.S. southern border.
They added that Trump had time to straighten out his message for the long slog to the November 2020 election.
"We've got a long way to go," said one Trump confidant.
(Reporting by Steve Holland; Additional reporting by Jeff Mason and Makini Brice; Editing by Kieran Murray and Peter Cooney)