A Fox News poll suggests US President Donald Trump trailing former vice president Joe Biden and no fewer than four other Democratic contenders as early campaigning for the 2020 election begins to gain steam.

A separate survey of battleground states, by CBS, shows Democrats strongly favour Mr Biden as the candidate most likely to beat President Trump in next year's elections.

The Fox poll showed Mr Biden leading President Trump by 49% to 39% among all registered voters nationwide, while Senator Bernie Sanders held nearly the same advantage over the president, at 49% to 40%.

Holding edges of one or two points over Mr Trump, albeit within the poll's three-point margin of error, were Senators Elizabeth Warren and Kamala Harris, as well as Mayor Pete Buttigieg of South Bend, Indiana.

The polling comes more than 500 days before the 3 November 2020 election, an eternity in the political world.

The president does not officially launch his re-election campaign until Tuesday, at a rally-style event in a huge arena in Orlando, Florida.

Three-quarters of Democratic voters believe Joe Biden is best positioned to defeat Donald Trump in 2020 

Still, the Fox poll, which was conducted between 9 June and 12 June, is seen as heartening by Democrats eager to chip away at President Trump's popularity, particularly in key battleground states such as Pennsylvania and Wisconsin.

Mr Trump's campaign recently dismissed leaked data from its own pollsters showing Mr Biden with double-digit leads in battleground states.

The campaign at first denied the data, but then acknowledged it, branding it as "ancient" because it dated from March.

Analysis - 2020 Vision: Donald Trump to launch re-election bid

But the new CBS poll confirms a clear Biden lead in battleground states among Democratic voters, as the crowded race for that party's nomination begins to take shape.

A belief among Democratic voters that Mr Biden is best positioned to defeat Mr Trump in 2020 was cited by three-quarters of Democrats as a decisive factor in their support.

The CBS News/YouGov Battleground Tracker survey, conducted between 31 May and 12 June, said Mr Biden had the backing of 31% of Democratic primary voters in 18 key early-voting states.

Mr Biden was trailed by senators Elizabeth Warren (17%), Bernie Sanders (16%) and Kamala Harris (10%).

Bernie Sanders insisted the survey showed he was the strongest candidate to defeat President Trump

The poll, with a 1.5% margin of error, looked at states including Iowa, New Hampshire and South Carolina, which hold primary elections in February, at the top of the electoral calendar, as well as states in the upper Midwest, where Mr Trump eked out narrow but decisive victories in 2016.

Ms Warren has been steadily rising in the polls, only recently reaching statistical equivalency with Mr Sanders, whose support has been slipping.

Mr Sanders has acknowledged that "polls go up and polls go down", but insisted that the survey showed he was the strongest candidate to defeat President Trump.

"I think we can win in Pennsylvania, Wisconsin and Michigan and some of the other battleground states," the self-styled democratic socialist said on "Fox News Sunday".

Democrats begin more earnestly winnowing down their field of nearly two dozen candidates when they hold successive nights of televised debates next week.

Representative Alexandria Ocasio-Cortez, the 29-year-old New Yorker who has emerged as a heroine to young progressives, suggested that Democrats could be in trouble in 2020 if they fail to nominate an energising candidate with working class appeal.

She said she would support 76-year-old Mr Biden if he wins the nomination, but added on ABC that "we have to really factor in the enthusiasm of voters ... an issue that we had in 2016".

"We need to pick a candidate that's going to be exciting to vote for - all people, women, people of all genders, races, income levels."

But the Fox poll found that Democratic voters, by roughly three-to-one, wanted a nominee who would provide "steady, reliable leadership" rather than a "bold new agenda".