Former President Obama considered the "weapon" most powerful in the election ammunition arsenal of Biden and Pennsylvania is the state he chose to deploy.

Former President Barack Obama has criticized his successor Donald Trump during his first live appearance at Democratic candidate Joe Biden's campaign.

"Donald Trump will not suddenly stand up for us all," Obama said at a "car rally" in Philadelphia on October 21.

At the same time, the former US president also warned Democrats to avoid complacent about the results of the polls, although the survey showed that Mr. Biden is maintaining an advantage in Pennsylvania.

"I'm not interested in surveys. There were a lot of polls in the past and it didn't work," Obama said.

With less than two weeks to go until the official election day, the arrival of former President Obama in the state that Trump won in 2016 underscores the importance of Pennsylvania.

This year's election saw both candidates work hard on Pennsylvania, from making constant visits to pouring tens of millions of dollars into advertising there.

The former president is seen as the Biden candidate's key representative, who, when giving his prime-time speech at the Democratic National Conference in August, appeared in the fundraising video and urged supporters.

However, the direct appearance of former President Obama, who is said to have the ability to attract top voters and is the most popular speaker of the Democratic Party, may help the Democratic base of support,

"One is that without Pennsylvania, President Trump would have no path to 270 electoral votes. The other is that former President Obama is still very popular in the state," said John Fetterman, Pennsylvania's deputy governor and a member of

Mr. Obama also proved energetic when he returned to the main stage to deliver criticism against President Donald Trump's way of managing the country and his personal behavior.

"Biden will not spoil the test program. He will not call the scientists a fool. He will not host a super infectious event in the White House," Mr Obama said.

Obama's way of organizing events is also different from Trump.

Candidate Joe Biden did not participate in the event.

Before the live campaign in Philadelphia, Mr. Obama urged supporters to vote and avoid taunting the country's problems.

"President Obama's ability to campaign for Democrats has only one opponent, Trump. Because Obama is the one who inspires us and says 'we can', while Trump is the one who dares to say that.

According to Holly Otterbein, editor of Politico, Democrats use their top campaigners to appeal to black voters, especially during his first live appearance this year,

"We will become a battlefield state and I think President Obama's visit reflects that," said Cherelle Parker, the majority leader in the Philadelphia City Council and the local representative for the campaign.

A survey by POWER Interfaith, a progressive group based in Pennsylvania, this month found that 72% of black male voters in Philadelphia are under 50 and 91% of black voters over 50 support Biden.

Former President Obama won in Pennsylvania twice, thanks in part to the enthusiastic support of African-American voters.

However, David Nakamura, editor of the Washington Post, said that Obama's arrival in Philadelphia was also a reminder that he was unable to help candidate Hillary Clinton win the state in 2016.

"If Obama can't shoulder Pennsylvania for Hillary, I have a lot of doubts as to what he can do for Joe Biden," said Charlie Gerow, a Republican political strategist in Pennsylvania.

But Obama said he never lost hope in Pennsylvania.