Tuesday is a key moment in this United States presidential election cycle. After all, they don't attach the prefix "Super" to just anything in the US.

In the Republican race, Donald Trump is expected to do very well. The frontrunner is ahead in eight of the 11 races and sitting second in the others.

That is a good place to be, especially in states where delegates are awarded based on the proportion of the vote won by a candidate.

And this is important. In the states with the highest number of delegates on offer - Texas, Alabama, Georgia and Tennessee - you have to win at least 20 percent of the vote to get any delegates at all.

And if the polls are to be believed, Marco Rubio, John Kasich and Ben Carson are below that threshold in all four of those southern states.

Ted Cruz is below in three, with only Texas, the senator's home state, saving him from a potentially huge embarrassment. He spent time and money in the these states believing evangelicals and "strongly Christian" voters would carry him to victory.

But Trump's momentum has been so strong that Cruz is desperately trying to shore up support in Texas. Anything less than a win there, and he is finished.

The final voting numbers are going to tell us a few interesting things.

First of all, no matter how well he does, Trump can't win the Republican nomination on Tuesday. But strong returns in all states and territories will tell us whether his support is as widespread as the polls are showing.

REPORTER'S NOTEBOOK: Why Chris Christie is endorsing Donald Trump

We will also find out if Cruz's early support is melting away. To be realistic, if he can't do well in the states that vote on Tuesday, he's not going to do well in the industrial Midwest.

Among the other candidates, Rubio will discover if he is truly cementing his position as the anti-Trump candidate. He is banking on Florida later in the month - a winner-takes-all state - to be his big victory.

But he is behind in the polls and not sure of victory there. There is a danger he could lose every state on Tuesday, and there are only so many times you can claim "victory" from second place.

And we will also discover if the recent attacks on Trump have had any effect.

The realisation that he is now the dominant candidate and in prime position to secure the domination has forced the other candidates to do what most of them refused to do for months, attack the billionaire businessman head-on.

They have gone after his business record, his business school being sued, and the fines he received for employing undocumented migrants. And on Sunday a new line of attack opened up when Trump initially refused to disavow the support of David Duke, a former head of the racist Ku Klux Klan.

Trump's rivals have been backed up on social media by senior conservative figures and Republican heavyweights, alarmed at the prospect of a Trump win.

But there are questions over how many people will pay attention and if the hashtag #NeverTrump will make a difference.

It might all be too late. Tuesday could be super for Donald Trump.