No other state leaves so many of its delegates unbound - allowed to vote for whomever they please at the convention. That's because it conducts its loophole primary in two parts. First is the "beauty contest", which is a presidential primary preference vote. The winner of the beauty contest gets all Pennsylvania's 17 at-large and bonus delegates. Donald Trump speaks at an election night event in New York. Credit:Bloomberg But the remaining 54 - the three delegates awarded to each congressional district - are unbound and elected in the delegate selection primary. In this part, voters directly elect delegates to the national convention. What makes Pennsylvania's Republican delegate selection primary so distinctive is that the ballot includes no guidance on whom a delegate will support at the national convention. (Prospective Democratic delegates commit to a candidate.) A voter will just see a list of names - some of whom might be recognisable, but others might as well be Joe Schmo. (It's called a loophole primary because it could circumvent the delegate allocation rules. Voters could, in theory, elect a slate of delegates who unanimously support one candidate, even in cases where a winner-take-all contest was prohibited.) The result is that Pennsylvania, with the seventh-most Republican delegates of any state, ranks 49th in pledged delegates. It's behind even Puerto Rico and the District of Columbia.

The other two states with loophole primaries - West Virginia and Illinois - take a somewhat different approach: The delegates pledge whom they'll support, and their preferences are listed alongside their names on the ballot. Trump surrounded by his supporters. Credit:AP When you read articles about whether Trump can reach 1237 pledged delegates by the convention, these 54 Pennsylvania delegates are held out of the analysis. Trump would be favoured to win a majority of Pennsylvania's 71 delegates under any other primary system. He leads by at least 9 percentage points in every survey in the state and is ahead of Ted Cruz by an average of 44 per cent to 28 per cent, according to The Huffington Post Pollster. He's up by a similar margin of 46 per cent to 30 per cent in The New York Times' demographic-based model, which also gives him an edge in 15 of the state's 18 congressional districts. Republican presidential candidate Donald Trump. Credit:AP

In a standard proportional allocation, Trump would probably be on track to win at least 40 of the state's delegates. In a winner-take-most system, like Indiana's or California's, Trump would be favoured to win at least 60 delegates. The difference matters a lot because Trump's prize of 1237 hangs on a thread. He is probably on track to finish a bit short of 1237. The Times' model puts his projected delegate deficit as slightly less than the number of delegates left unpledged (54) in Pennsylvania, meaning he would be projected to win if they were bound delegates. Trump supporters in Washington. Credit:AP Although facing a challenge in Pennsylvania, Trump has benefited from other quirks in the Republican primary rules. Compared with the Democrats' rules, the Republican ones are far more favourable to a candidate who wins with a plurality of the vote. And with the race as close as it is, any number of tweaks in the rules could make Trump a clear favourite - or an overwhelming underdog. New York Times