PHILADELPHIA (REUTERS) - Former US Vice-President Joe Biden on Saturday (May 18) will hold a presidential-style rally intended to make his march towards becoming the Democrat to take on President Donald Trump seem inevitable, even as rivals search for ways to slow him down.

Since entering the race last month, Mr Biden, 76, has largely ignored the other 23 contenders in the Democratic field, instead training his fire on Republican Trump.

Mr Trump, in turn, has regularly knocked Mr Biden, making the 2020 presidential contest sometimes feel like a general election more than a year before the vote takes place.

Mr Biden's outdoor rally in Philadelphia, where he has established his campaign headquarters, illustrates the importance of Pennsylvania to Democratic hopes next year. Mr Trump narrowly won the state over Mrs Hillary Clinton in 2016.

After Mr Biden leaves, Mr Trump will hold an event of his own on Monday in the northeast part of the state.

Mr Biden will not have the luxury of shrugging off the rest of the Democratic field much longer. While opinion polls show him with a substantial lead, other candidates have begun targeting him.

An emerging antagonist has been US Senator Kamala Harris, who this week mocked calls by Biden supporters that she join him on the Democratic ticket as vice-president. Ms Harris said it should be the other way around.

"I think that Joe Biden would be a great running mate," Ms Harris told reporters. "As vice-president, he's proven that he knows how to do the job."

Mr Biden, a US senator for 30 years and a two-term vice-president under Mr Barack Obama, has argued he is best positioned to take on Mr Trump next year.

The cheeky remark by Ms Harris underscored the tension that runs through the Democratic Party as its activist wing grapples with the notion of nominating a moderate white male such as Mr Biden rather than a progressive woman such as Senator Elizabeth Warren or person of colour like Ms Harris.

MIDDLE OF THE ROAD?

Democratic nominating contests begin next February, giving the dynamics of the race plenty of time to shift. But Mr Biden has opened up a more than 20-point lead over his nearest rival, Senator Bernie Sanders, in several public opinion polls.

In New Hampshire this week, Ms Harris took issue with Biden's assertion that a sweeping 1994 crime bill Mr Biden backed in Congress did not lead to mass incarceration of prisoners.

African-Americans, a key voter demographic for both Mr Biden and Ms Harris, have been particularly critical of the legislation, saying it devastated black communities.

Ms Warren has criticised Mr Biden's support of the credit card industry while a senator. Mr Sanders has blasted Mr Biden's past support of the North American Free Trade Agreement and the Iraq War.

US Representative Alexandria Ocasio-Cortez, who is not running for president but holds influence over young progressive voters, appeared this week to criticise Mr Biden after Reuters reported that he likely would advance a plan to tackle climate change less sweeping than Ms Ocasio-Cortez's 'Green New Deal'.

Mr Biden pushed back at Ms Ocasio-Cortez, saying he has never been "middle of the road" on the climate issue.

"There are very loud voices on the very new progressive side of the agenda, and I think it's useful," Mr Biden said in New Hampshire.

"I think they're good. They're smart people, and they should be able to be making their case."

At his Philadelphia rally, his biggest campaign event yet, Mr Biden is expected to speak in broad policy outlines and call for national unity.

He was ridiculed by some liberal commentators this week for similar talk, suggesting Republicans would have an "epiphany" and begin cooperating with Democrats once Mr Trump is out of office.