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The United States may be a democracy, but the party presidential nomination process - upon closer inspection - is hardly a shining beacon of democratic light.

For most of US history, party nominees have been decided by political power brokers and deal-makers behind closed doors. Parties operate like private clubs - they make their own rules and are suspicious of outsiders.

Only in recent history has a more open system of primaries and caucuses been grafted onto the process to give the average American a say in who appears on the general election ballot.

In a close, contentious primary season, however, the veneer of accountability can rub off, exposing the sometimes unsightly gears that still power the US political system.

This has prompted objection from the supporters of two candidates in particular - Donald Trump and Bernie Sanders - who feel that the party establishments are arrayed against their presidential quests.

But are their concerns valid? Here are answers to four pertinent questions as the nomination battles approach their final months.

Is Trump being cheated?

34 Number of delegates Ted Cruz received in Colorado, which held conventions to pick delegates 743 Trump total delegate count

545 Cruz total delegate count AP

Mr Trump is leading the race for the Republican nomination, but it's starting to feel like he's not winning.

While he's comfortably ahead, with 743 delegates to second-place Ted Cruz's 545, there's mounting evidence that he's being outmanoeuvred in a behind-the-scenes political process that could come into play if he doesn't reach the magic 1,237 delegate number necessary to secure the nomination outright.

In Colorado - which selected its delegates last week at party gatherings instead of through primaries or caucuses - Mr Cruz walked away with all 34 delegates. Even in states that have held contests won by Mr Trump, Mr Cruz's team has been working doggedly to ensure that their people become delegates.

While Mr Trump swept South Carolina's 50 delegates, for instance, the state's convention delegation will be riddled with Cruz supporters who, while bound to Mr Trump on the first few ballots, can switch to the Texas senator if there is a protracted convention battle.

It has Mr Trump and his people crying foul.

"This is happening all over our country - great people being disenfranchised by politicians," Mr Trump tweeted on Monday. "Repub party is in trouble!"

Paul Manafort, Mr Trump's new aide in charge of managing the delegate-selection process, accused the Cruz campaign of using "Gestapo tactics, scorched-earth tactics" in Colorado.

If, as Mr Trump asserted on Monday, the system is "rigged" and "crooked", however, it isn't always tilted in favour of Mr Trump's opponents. Thanks to the Republican Party's delegate-apportioning system, including Florida's winner-take-all primary, Mr Trump has secured a larger share of the delegates so far (45%) than he has of the raw primary vote (37%).

If Mr Cruz manages to win the nomination at the Republican convention despite trailing Mr Trump in total delegates and share of the popular vote, Mr Trump may have reason to feel aggrieved.

But before he complains too loudly, he might want to heed some sage advice attributed (incorrectly) to Albert Einstein: "You have to learn the rules of the game. And then you have to play better than anyone else."

Mr Trump should be familiar with the quote, since he tweeted it in October 2014.

Why isn't Sanders catching up?

Mr Sanders has won five state contests in a row and seven of the last eight. If he were an American football team he'd be poised for the playoffs. If he were prize fighter, he'd be tuning up for a title bout.

Instead his pledged-delegate deficit to Mrs Clinton has gone from daunting to only fractionally less daunting. Over the course of his recent run, the Vermont Senator has picked up a net of just 91 delegates, despite winning Wisconsin 56% to 43%, Utah 79% to 20% and Washington 72% to 27%.

According to a New York Times calculation, the former secretary of state currently has 1,305 pledged delegates, while Sanders has 1,086. Add in the non-binding support of "super-delegates" - Democratic officeholders and party functionaries who also cast ballots for the nominee at the convention - and Mrs Clinton's lead balloons to 1,774 to 1,117.

To secure the Democratic nomination without drama at the convention a candidate needs the backing of 2,383 delegates

The problem for Mr Sanders is that while he's been posting sizeable wins over the past month, they've largely been in delegate-poor states, like Wyoming (14 delegates), Idaho (23) and Alaska (16). His successes pale in comparison to Mrs Clinton's massive earlier wins in populous southern states like Texas, Florida and Georgia, which alone netted her 184 delegates over Mr Sanders.

If Mrs Clinton performs as expected in the coming contests in New York (291 delegates), Maryland (118) and Pennsylvania (210), she'll largely erase all the modest ground Mr Sanders has made up over the past three weeks.

Does the popular vote even matter?

9,412,426 Votes for Hillary Clinton during primary season so far 7,034,997 Votes for Bernie Sanders during primary season so far Getty Images

If delegate maths and selection rules make your head hurt, at least we can rely on the raw vote totals to get a feel for how popular the remaining candidates are, right?

Wrong.

According to current tabulations Hillary Clinton has received 9,412,426 votes during the primary season so far. Bernie Sanders has received 7,034,997. That 2.4m vote lead has been relentlessly touted by the former secretary of state and her supporters to counter claims from Sanders' faithful that their man is more popular than the delegate tallies indicate.

Some states that hold caucuses - like Iowa and Washington - aren't included in that number, however, because they don't report vote totals.

The Washington Post's Glenn Kessler tried to extrapolate numbers for the remaining states based on their total voter turnout and concluded that Hillary Clinton leads Bernie Sanders by 2.3m votes - still a significant margin.

Among the Republicans, who are better at providing full vote totals, Mr Trump leads with 8,256,309 votes. Mr Cruz is second (6,319,244) and former candidate Marco Rubio is third (3,482,129), followed by Ohio Governor John Kasich (2,979,379).

In the end the popular vote may give the leading candidates a claim of legitimacy as the people's choice - but appearances can be deceiving.

"The media has created the perception that the voters choose the nomination. That's the conflict here," North Dakota delegate Curly Haugland told a television interviewer. "The rules are still designed to have a political party choose its nominee at a convention. That's just the way it is."

Can Republican convention delegates be bribed?

"Well, there's the law, and then there's ethics, and then there's getting votes. I'm not going to get into what tactics are used." Donald Trump adviser Paul Manafort Reuters

If, as appears to be increasingly likely, the Republican primary season ends without Donald Trump securing the 1,237 delegates needed to secure the nomination, the Republican National Convention could turn into a political free-for-all unrivalled in modern US political history.

After several rounds of deadlocked balloting, most convention delegates would be free to vote according to their conscience. But could that conscience be nudged by, say, a free weekend at a Donald Trump golf resort, a nice dinner with the Cruz family or even a choice spot in a John Kasich administration?

Maybe! While there are detailed anti-corruption laws governing the behaviour of public officeholders, convention delegates are private citizens. While government regulations prohibit them from taking money from corporations, labour unions, government contractors or foreign nationals, beyond those restrictions the law is much murkier.

Campaigns and their wealthy donors could likely cover delegates' travel expenses, no matter how lavish. Gold watches? Bags of small, unmarked bills? Who knows? State anti-bribery laws may apply, but there's scant legal precedent.

Perhaps the greatest deterrent to untoward action by campaigns is the negative publicity such naked attempts at influence could have if they're documented. But public perceptions and attitudes this political season has been difficult to predict, to say the least.

Although the national convention is still months away, accusations of dirty tricks have already started flying. On Sunday, Mr Trump took to Twitter to accuse the Cruz campaign of misdeeds during the South Carolina state party convention - a charge Mr Cruz vehemently denied.

"I win a state in votes and then get non-representative delegates because they are offered all sorts of goodies by Cruz campaign," Mr Trump wrote. "Bad system!"

During a television interview that same morning, however, Trump adviser Paul Manafort appeared to acknowledge that his campaign won't be shy in wooing delegates at the national convention.

"Well, there's the law, and then there's ethics, and then there's getting votes," he said. "I'm not going to get into what tactics are used. I happen to think the best way we're going to get delegates is to have Donald Trump be exposed to delegates, let the delegates hear what he says."

Another Trump adviser, Barry Bennett, said they wouldn't be offering "seats on the Trump airplane or anything like that".

"There's obviously a big line - we're not going to do anything immoral, illegal or unethical," he said.

But when a presidential nomination is at stake, and it comes down to just a handful of delegates, that "big line" may end up looking awfully fuzzy.