Democratic strategists have been reassessing the chances of Bernie Sanders winning the party's presidential nomination and concede he does have a chance of being the candidate who goes up against Donald Trump next year.

Sanders' health problems - he was sidelined for a week after having a heart attack - and Elizabeth Warren's rise in the polls had some people writing off the Vermont senator's campaign.

But Sanders has proven remarkably resilient - bouncing back on the campaign trail and turning in strong debate performances, which is strategists second thoughts.

Democratic strategists have been reassessing the chances of Bernie Sanders winning the presidential nomination

Strategist concede Sanders could win the right to take on Donald Trump in 2002

Several of them told Politico that Sanders could be in it to win it, making it all the way to end to be the last challenger standing to front runner Joe Biden.

'It may have been inevitable that eventually you would have two candidates representing each side of the ideological divide in the party. A lot of smart people I've talked to lately think there's a very good chance those two end up being Biden and Sanders,' said David Brock, a longtime Hillary Clinton ally. 'They've both proven to be very resilient.'

In 2016, Sanders mounted a strong presidential campaign that surprised observers with its strength in fundraising and primary wins.

Clinton eventually won the nomination but Sanders held strong and didn't endorse her until July 12, a few weeks before she took the stage at the Democratic National Convention to accept the party's nomination.

'Our job is to do two things: to defeat Donald Trump and to elect Hillary Clinton ... It is easy to boo, but it is harder to look your kids in the face if we are living under a Trump presidency,' he said at the time.

In the race for the 2020 nomination, Sanders is running second to Biden in the RealClearPolitics average of polls on the race. The Vermont senator has 19 per cent to the former vice president's 28 per cent.

He has stayed in the race while rivals like Kamala Harris and Beto O'Rourke, who were seen as having strong candidacies, dropped out.

Sanders has proven he can raise the funds to keep his campaign rolling. He led the third-quarter in fundraising among the candidates, hauling in $25.3 million to Warren's $24.6 million. Biden came in fourth with $15.2 million while Pete Buttigieg raised $19.1 million.

Sanders is running second to Joe Biden in national polls and has overtaken Elizabeth Warren

Some moderates say Bernie Sanders - seen in California last week with Alexandria Ocasio-Cortez, is too liberal to win the nomination

The Vermont senator uses the same fundraising tactic he used to effectiveness in 2016: bringing in small-dollar donors who can give repeatedly as they don't hit their fundraising limit while eschewing high-dollar fundraisers.

'I believe people should take him very seriously. He has a very good shot of winning Iowa, a very good shot of winning New Hampshire, and other than Joe Biden, the best shot of winning Nevada,' Dan Pfeiffer, a former adviser to President Barack Obama, told Politico. 'He could build a real head of steam heading into South Carolina and Super Tuesday.'

Sanders won the New Hampshire primary in 2016 and he leads in the RealClearPolitics average of polls in the 2020 primary.

Moderate Democrats, however, still doubt Sanders can win the nomination and question his electability. Sanders has pushed liberal programs like Medicaid for all, free college tuition and the Green New Deal environmental plan.