Is Marco Rubio's bid for the Republican presidential nomination over? With poor showings in the four states that voted on Tuesday, it could be.

Republican primaries were held in Idaho, Michigan, and Mississippi, in addition to a caucus in Hawaii. Businessman Donald Trump won three of the four states, while Texas Senator Ted Cruz prevailed in Idaho.

Rubio, a senator from Florida, did no better than 16 per cent of the vote in any state, finishing in fourth place in Michigan and Mississippi and is likely to come out of the night without a single delegate to his name. His opponents did better — according to the New York Times, Trump won 71 new delegates across the four states, with Cruz taking 56 and Ohio governor John Kasich winning 17 on the strength of a strong showing in Michigan.

The case for Rubio, who quickly garnered the support of the Republican Party establishment after former Florida governor Jeb Bush bowed out of the campaign, is increasingly more difficult to make. And the math suggests that any hope of Rubio winning a majority of delegates by the end of the race is extremely slim.

That Rubio performed so poorly does not come as much of a surprise, as his polling numbers were low in both Michigan and Idaho. But his position was looking much better only a week ago, when he was in second place and polling around 18 per cent in Michigan. In the end, he finished with just nine per cent of the vote in the state.

Michigan was more of a surprise on the Democratic side, where Vermont Senator Bernie Sanders won despite trailing Hillary Clinton in the polls by about 20 points. But with a landslide victory in Mississippi, Clinton can easily afford to narrowly lose a few states to Sanders in the North if she romps to huge victories in the South.

Grim future for Rubio

Clinton's advantage is due to the rules of the Democratic nomination, which award delegates proportionate to the share of the vote each candidate receives. The manner in which delegates are awarded in the Republican race, however, varies from state to state. This has a big impact on how the next stage of the campaign could unfold.

Five big states are voting on March 15: Florida, Ohio, North Carolina, Illinois, and Missouri. The first two are winner-take-all states, where the candidate who finishes in first place takes every delegate. The last two are winner-take-most states, where the candidate who wins the state tends to take all or almost all of the delegates on offer.

North Carolina awards its delegates proportionately.

Rubio's goal has long been to win his home state of Florida. But the latest polls suggest he is trailing Trump by between 15 and 20 points there, and in fact Rubio has never led in the state since he threw his hat in the ring.

Rubio is also behind by double digits in Illinois and is in third in North Carolina. He isn't in the running whatsoever in Ohio, where Kasich is nipping at Trump's heels.

The run of poor showings by Rubio is likely to continue into next week. If he loses Florida, he may drop out of the race. 

Contested convention? Maybe not

With Trump leading in the polls nationwide and winning state after state, the Republican Party establishment has been banking on a contested convention to oust him. This could occur if Trump fails to win a majority of delegates, giving the convention's attendees the opportunity to choose another candidate.

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Donald Trump is the front-runner for the Republican presidential nomination. (Reuters)

But the prospect of a contested convention could quickly diminish. Trump did not win a majority of delegates on Tuesday, but the next set of primaries and caucuses could set him on course to pass the majority threshold.

If Trump wins the winner-take-all or winner-take-most states of Florida, Illinois, Missouri, and Ohio, while also finishing strong in North Carolina and the other areas that will be casting ballots over the next week (the Virgin Islands, Washington, D.C., and the Northern Marianas), he could emerge with a wide lead over his rivals.

With these victories, Trump could end up with a total of around 800 delegates after March 15. Cruz might stand in second place with around 380, Rubio in third with around 170, and Kasich in fourth with about 65. 

That would give Trump more delegates than all of his opponents combined. The only way to prevent that would be for Rubio to win Florida or for Trump to lose at least a combination of two states among Illinois, Ohio, and Missouri. 

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The potential for a contested convention still exists if Trump falters on March 15, giving Rubio and Kasich reason to stay in the race and keep Trump from achieving a majority. But if Trump runs the table next week, or nearly does so, the nomination may be as good as his.

It will also likely end Rubio's campaign. Officially, at least. Tuesday's results might have ended it already.