In any other year, the question of whether the now presumptive Democratic Party nominee Hillary Clinton could win the November presidential polls would be academic.

Pit a former first lady, former senator and former secretary of state with a well-oiled national campaign machinery against a brash, polarising businessman with no experience and little organisational capacity, and one would normally assume that it will end in a landslide victory for the former.

But this year is not any other year. This is the year when almost every bit of conventional wisdom about politics has been upended: one in which a reality TV star easily emerged atop what many considered the most talented field of Republican candidates in a generation; one in which a 74-year-old grumpy self-declared socialist became the icon of millions of millennials.

Even given the topsy-turvy nature of the primary elections that have just ended, most still expect Mrs Clinton to be able to pull off a victory. But she will have to overcome some major obstacles to get there.

THE HURDLES AHEAD

The unusual mood of the current American electorate may well prove to be the most complicated puzzle for Mrs Clinton to solve.

The electorate this year has shown a clear disdain for the establishment and there is perhaps no candidate more "establishment" than the former secretary of state.

Throughout the primaries, she has seen her rival, Senator Bernie Sanders, capture all the energy and enthusiasm of young voters. He was the shiny new thing that excited voters, while she was seen as a representative of the party's past.

Trying to change that perception is not easy because Mrs Clinton has been on the scene for so long.

While voters can project all manner of hopes on an upstart candidate like Senator Barack Obama in 2008 or Mr Sanders this year, with Mrs Clinton, voters already think they know what they are going to get.

Her public image as a hard-nosed career politician is so entrenched in the minds of voters that it supersedes the fact that she may well be the first-ever female president of the US. When people look at Mrs Clinton, they see a politician first and a woman second.

It remains to be seen how much goodwill she will gain from the historic significance of possibly being the first woman to be elected to the White House, but it is worth noting that Mrs Clinton has not often played up that point.

Tied in with the reputational problem is the fact that what people already think of her as a politician isn't always very flattering.

Recent polls show that less than one-third of voters think that she is trustworthy, a score lower than that of Mr Trump.

Mr Trump has seized on this in recent weeks, labelling her "crooked Hillary", and how Mr Trump plays this point during televised debates with Mrs Clinton will be something to watch. The tycoon has shown a propensity for hitting opponents hard when he detects a weakness.

Throw in a State Department e-mail scandal - she used private e-mail for official business while she was secretary of state - and the trust issue is nearly certain to loom over the rest of her campaign.

All this makes it difficult for Mrs Clinton to unify the party and draw in supporters of Mr Sanders.

The depth of the disdain for Mrs Clinton from this liberal faction of the party should not be under-estimated. Every primary election is divisive and controversial but few have featured the sort of behaviour we saw this year.

In April, Sanders supporters in California showered Mrs Clinton's motorcade with dollar bills as she drove to a fund-raiser with actor George Clooney, to mock her closeness to big corporate donors. Last month, those backing the senator threw chairs, booed and made death threats to members of the Nevada Democratic Party over what they saw as moves that unfairly favoured Mrs Clinton.

And last week, Sanders fans flooded the Facebook page of progressive Senator Elizabeth Warren, accusing her of selling out by endorsing Mrs Clinton. "An endorsement of Hillary Clinton will undo everything for which you have stood. An endorsement of Hillary Clinton will undo my faith in you as a person with integrity," wrote one.

Then there is the one factor working against Mrs Clinton that she really has little control over.

In two centuries of US elections, what she is trying to achieve this year - retain the White House for a party that has had it for two terms - has been historically very difficult.

In seven of the last nine elections, voters have decided to switch parties controlling the White House after two-term presidents. The last time Democrats were able to win three times in a row was in 1940.

STILL BOOKMAKERS' FAVOURITE

It should be noted, of course, that Mrs Clinton is still the bookmakers' favourite to win. The 68-year-old does have a lot going for her - chiefly because she is coming up against a guy named Donald Trump.

The two rivals are both struggling to unite their party base. But unlike Mr Trump, Mrs Clinton will have a unified party leadership behind her. Once the primary race was effectively over, the party leadership quickly fell in line behind her. President Obama, Vice-President Joe Biden, Senator Warren and House Minority Leader Nancy Pelosi all endorsed Mrs Clinton within a day of one another. And it is largely expected that Mr Sanders will eventually do the same thing.

Contrast that with the way the Republican establishment has reluctantly dragged itself behind Mr Trump. House Speaker Paul Ryan had to be coaxed slowly into getting behind the nominee, while major party figures, like former president George W. Bush and former nominee Mitt Romney, have declared that they cannot back Mr Trump.

This need for unity at the top isn't just symbolic, it affects the sort of machinery the candidate can tap into.

And Mr Trump is starting off with a bit of logistical disadvantage. Mrs Clinton has a superior donor base and ground game. She has more than 800 campaign staff to her name and hundreds of millions of dollars in her campaign war chest.

With the help of the party, she will have access to thousands more volunteers and voter databases across the country. That is going to be invaluable for the nitty-gritty of the election campaign, knocking on doors, making calls and wooing undecided voters.

Mr Trump has only a tenth of the number of staff that Mrs Clinton has. He has few field offices and an inadequate fund-raising machinery. He is independently wealthy but even he cannot afford the close to US$1 billion (S$1.36 billion) it is estimated to take to win a modern election. The free-wheeling style that worked in a primary election where candidates campaign in one state at a time isn't going to work in a nationwide election.

Coming up against an abrasive figure like Mr Trump also reduces concern that independent Sanders voters will switch to the other side. Mr Trump is a sufficiently toxic figure for the Sanders base that even the angriest cannot imagine themselves voting for the businessman.

THE GAME CHANGER

If on balance, Mrs Clinton has a slight edge, there is one final thing to consider: an exogenous shock to the system.

A terrorist attack a week before the polls, a deep economic crisis, a health scare for Mrs Clinton or an indictment over her handling of State Department e-mails can derail even the best-laid plans.

All of this thus adds up to a changeable general election season that is perhaps fitting of everything that has happened so far.

It may well end the way many are predicting it will - with the first-ever female American president - but the only thing everyone can say with any certainty is that the coming five months are hard to predict.