Can Clinton pull off a hat-trick of Democrat wins?

President Harry Truman was part of an era of Democratic dominance

Not since the era of Franklin Delano Roosevelt and Harry S Truman have the Democrats won three presidential victories in a row.

The 2016 election presents the party with a rare opportunity to pull off a historic hat-trick.

America's political geography gives the Democrats an enormous advantage.

So, too, does the country's changing demography, because constituencies that favour the Democrats are growing in electoral influence.

Despite the Republicans' current strength in congressional and gubernatorial politics - presently, the GOP holds the House of Representatives and the Senate, along with 31 governors' mansions - the party is weak in presidential politics.

It has lost four of the past six presidential elections. In five of those, the Democrats have won the popular vote.

The "blue wall" is especially advantageous.

That is the name given to the 18 states, as well as the District of Columbia, that have voted Democrat in every presidential election since Bill Clinton's first victory in 1992.

Democratic Blue Wall:

California (55), Connecticut (7), Delaware (3), Hawai'i (4), Illinois (20), Maine (4), Maryland (10), Massachusetts (11), Michigan (16), Minnesota (10), New Jersey (14), New York (29), Oregon (7), Pennsylvania (20), Rhode Island (4), Vermont (3), Washington (12), Washington DC (3), Wisconsin (10).

What makes the blue wall such a towering edifice is the size of its building blocks: some of the country's most populous states, like California, New York, Illinois, Michigan and New Jersey.

To win the Electoral College, the institution that elects the president on a state-by-state basis, the victorious candidate requires 270 votes.

Strong core support

For the past six elections, the states that make up the blue wall have yielded 242, just 28 short of the target.

The Republicans have a wall of their own: 13 states that have voted for the GOP's presidential candidate in the past six elections.

But those states amount for only 102 Electoral College votes between them.

To some, then, the "Red wall" looks more like a flimsy picket fence.

Republican Red Wall:

Alabama (9), Alaska (3), Idaho (4), Kansas (6), Mississippi (6), Nebraska (5), North Dakota (3), Oklahoma (7), South Carolina (9), South Dakota (3), Texas (38), Utah (6), Wyoming (3).

The Blue Wall is by no means insurmountable.

Though it held firm at the 2000 and 2004 elections, George W Bush emerged the victor.

President George W Bush beat the blue wall

Many of these blue states, like New Jersey, Massachusetts and Illinois, have Republican governors, and the GOP has not given up hope of turning them red.

Pennsylvania, with its 20 Electoral College votes, is particularly high on the their target list.

But the wall does grant the Democrats an inbuilt advantage in the Electoral College.

Just consider this statistic. Since 1992, the Republicans have achieved an average of 211 Electoral College votes. The Democrats' average is 327.

Demographic advantage

Demographics also appear to favour the Democrats: the support they are now receiving from minorities, Millennials (voters under 30) and women.

The Democrats have opened up a huge lead among minority voters, a growing and increasingly important part of the electorate.

At the last presidential election, 71% of Latinos voted for Barack Obama, up from 67% in 2008. Some 73% of Asian-Americans also voted Democrat, along with 93% of African-Americans.

Younger voters, who tend to be more liberal-minded on issues like same-sex marriage and immigration, are also leaning towards the Democrats.

Some two-thirds of Millennials voted for Obama in 2012.

A majority of women have also favoured the Democrats in recent presidential elections. Fifty-five per cent of women voted for Obama in 2012, while the figure for unmarried women was even higher at 67%, partly because the Republican Party has become associated with restrictions on abortion.

Obviously all is not lost for the GOP, not least because the party has demographic advantages of its own.

In 2012, 59% of white voters plumbed for Mitt Romney. Among the so-called silent generation, those born between 1925 and 1945, the Republicans have a lead of 47% to 43%. But America is becoming less white, and that presents problems for the Republicans.

Their prime strategy since the civil rights era of the 1960s, after all, has been to target white voters, regardless of their income levels.

Obama support

Next year, the GOP will be hoping that the so-called "Obama coalition" of minorities, Millennials and women, does not become the "Hillary Coalition," if, as expected, she wins the Democratic presidential nomination.

Black voters will not turn out in such high numbers for Hillary, they reckon.

The GOP also hopes to make inroads into a Latino vote deterred from backing Republicans because of the party's tough line on immigration.

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Party strategists believe there is truth in Ronald Reagan's famous observation: "Latinos are Republicans. They just don't know it yet."

As for the Millennials, a string of recent polls suggest that their support for the Democrats is waning - although a survey conducted in April by the Harvard Institute of Politics suggested that 55% of voters under the age of 30 would prefer the White House to remain in Democratic hands.

There are Democrats who believe that the Hillary coalition could be even more formidable than the Obama coalition.

Campaigning to become America's first female president, she will hope to attract higher levels of support from white women, more than half of whom voted Republican in 2012.

She might attract more male white voters than Obama.

Yet Democrats run the risk of over-confidence, a mistake made by Republicans following the back-to-back victories of George W Bush.

In those heady days, strategists like Karl Rove spoke assuredly of an emergent permanent Republican majority, only to see Obama score two victories.

More recently, GOP morale has been boosted by the work of the political journalist John Judis, who predicted at the start of the century an "emerging Democratic majority".

In January, Judis penned a revisionist essay headlined "The Emerging Republican Advantage," which argued that the Republican triumph at last November's congressional mid-term elections was "the latest manifestation of a resurgent Republican coalition".

White vote

The Republicans were even more dominant among white voters, he observed, which was problematic for the Democrats because they still required between 36% and 40% of the white working-class vote to win the presidential election.

But it is always a mistake to equate strength in congressional politics with success in presidential politics.

President Reagan began an era of Republican dominance

Often the party with a lock on Capitol Hill has suffered a near lockout at the White House.

Between 1968 and 1992, for instance, the Democrats dominated the House of Representatives.

For that entire era, a Democrat sat in the powerful speaker's chair.

But during that era, the Democrats won just one presidential election, when Jimmy Carter edged out Gerald Ford in 1976.

The electorate that votes in congressional elections is different in size and make-up to that which turns out in presidential polls.

History suggests it will be hard to win three consecutive victories.

Since the war, the Republicans have only managed it once, when George Herbert Walker Bush followed Ronald Reagan into the White House.

Could Hillary Clinton do what no Democrat has done for more than 65 years?