How badly do Republicans want to win? What are they prepared to forgive? With three weeks to go until the first nominating contest of the 2016 election cycle, in Iowa, these are the key questions as the party seeks a candidate to stop Donald Trump. The three main contenders, Marco Rubio, Ted Cruz and Chris Christie are all deeply divisive figures, but one of them will have to do.

Trump is unpalatable to the establishment for many reasons - his three wives, his formerly Democratic positions, his shallow religious faith, and the stream of insults he has directed at his Republican colleagues - but chiefly because of the looming electoral catastrophe he represents.

If he can bluster his way through the primaries and win the nomination, he will almost certainly face Hillary Clinton in the presidential election, and while she has vulnerabilities, as the consummate insider and triangulator-in-chief, she is also a sharp, diligent and vastly experienced opponent.

Trump has alienated Latinos to a degree previously unthinkable even to a party that won just 29% of the Hispanic vote last time around. When Fox News moderator Megyn Kelly raised his record of making sexist remarks in the first Republican debate, he was able to deflect the charge, but against a woman running to be the first ever female president, his crass, boorish behaviour would be a huge liability.

The Grand Old Party’s power-brokers fear that Trump would not only suffer a historic defeat: he would take Republican candidates for the Senate and House of Representatives down with him, cutting into or even threatening the party’s legislative majorities.

In an aggregate of the latest national polls, Trump enjoys the backing of 38.6% of Republican voters, more than double his nearest rival, Cruz, with 16.3%, followed by Rubio at 10.7%. The party’s pragmatists will have to unite around a single candidate to beat him.

The bookies make Rubio the favourite to win the nomination, ahead of Trump, despite polls that show him trailing in all of the earliest states to vote. This is based on the party’s recent history of getting behind nominal moderates John McCain and Mitt Romney in the belief that they had the best chance in a general election. Given the depth of conservative hatred towards RINOs - Republicans In Name Only - the bookies may be wrong.

There is, as it happens, nothing moderate about Rubio’s platform. He is against abortion with no exceptions, even for rape or incest. On foreign policy, he is a neo-conservative hawk who would send American soldiers into the Middle East to spread democracy, notwithstanding the disastrous attempt to do so in Iraq.

If elected, he would further deregulate the financial system. The main plank of his economic policy is tax cuts for everyone. Economists estimate that his proposal would blow a $4 trillion hole in the deficit. The New York Times called it the “puppies and rainbows” plan.

Rubio’s great apostasy, and greatest weakness in the nomination contest, is his prior support for comprehensive immigration reform. In June 2013, he co-sponsored a bill in the Senate that provided a path to citizenship for undocumented immigrants. Time magazine put him on the cover and called him the Republican Saviour. When it became clear that the bill would die in the House of Representatives, Rubio washed his hands of it, but the damage was done.

On the campaign trail, the Senator from Florida tells his own family’s immigrant story. His Cuban parents fled in 1956, three years before Fidel Castro’s revolution, and in those more welcoming times were granted residency because they had relatives in the United States. His father was a barman, his mother a maid, but they encouraged their children to believe in the American dream of social mobility.

Rubio got into politics when he was twenty-six and still living at home with his parents, winning a seat on the West Miami city commission. He has since entered another five elections, and never lost.

As a gifted orator with a compelling personal story and telegenic looks, Rubio’s appeal to party strategists is obvious. Polls show that Latino voters regard him with scepticism, but his ability to make his case in Spanish is potentially a major asset. His Christian faith is so strong (and so central to his pitch) that he worships twice a week, attending an evangelical Protestant church on Saturdays and a Catholic church on Sundays.

At 44, he is bidding to become the third youngest president, after John F Kennedy and Teddy Roosevelt, and he stresses his youth at every opportunity, presenting himself as an Obama-esque figure of generational change that, given the chance, will show Hillary Clinton up as old and out of touch.

On October 30, Paul Singer, a hedge fund billionaire who gave more money to Republican candidates than anyone else last year, sent a letter to fellow plutocrats making the case for Rubio. The Senator has also worked hard to earn the support of Las Vegas casino magnate Sheldon Adelson.

A group calling itself the Conservative Solutions Project ran $5.5 million worth of adverts supporting Rubio last year, but because it is defined as a “social welfare” organisation, supposedly advocating a cause rather than a candidate, it does not need to disclose the identity of its donors. All we know is that some very rich people believe Rubio has a shot.

In the third Republican debate, Rubio put paid to the candidacy of his mentor and former friend Jeb Bush with a single devastating line. When Bush criticised him for missing Senate votes, Rubio responded: “someone has convinced you that attacking me is going to help you.”

This was what pundits call a ‘moment,’ endlessly replayed the next day. It summed up the desperately underwhelming campaign of the one time front-runner.

With Bush out of the way, it appears that Rubio’s main rival is Texan Senator Cruz, who leads the polls in Iowa, ahead of Trump, and is challenging for second and third elsewhere. Rubio is running a general election campaign in the primaries, with the aim of cornering establishment cash and endorsements. By contrast, Cruz is making his appeal to the party’s no-compromise right wing in the hope that if Trump drops out, he will inherit his supporters.

In a nomination contest that has favoured candidates with no political experience, Cruz presents himself as an outsider. “Some people think the Washington Redskins is an offensive name,” he tells conservative crowds. “There’s an easy way to fix that. You can just drop the word ‘Washington.’”

This is a bit rich coming from a Princeton and Harvard-educated lawyer who clerked for Supreme Court Justice William Rehnquist, worked for elite law and lobbying firms and was part of the legal team that persuaded the Supreme Court to award the 2000 presidential election to George Bush. Cruz had made strenuous, futile efforts to repeal the Affordable Care Act, but gets his own health insurance through Goldman Sachs, where his wife is a Vice President.

Cruz could recite the US Constitution by heart before he was out of short trousers, and throughout his career he has been recognised for a formidable and piercingly intelligent debater. He is also known for being, in the words of his Princeton roommate Craig Mazin, a “nightmare of a human being”.

It is hard to overstate how much Cruz is disliked on Capitol Hill, particularly by fellow Republicans that do not subscribe to his scorched earth approach to legislating. Cruz was one of the key engineers of the government shutdown in October 2013. He has called Majority Leader Mitch McConnell a liar on the Senate floor, and repeatedly offered his support to Tea Party groups seeking to oust Republican members of Congress considered insufficiently conservative.

Former House Speaker John Boehner called him a “jackass”. To McCain, he’s a “wacko bird”. His old boss George W. Bush told a group of Republican voters “I just don’t like the guy.” Cruz welcomes their hatred: “I wear it as a badge of honour because I refuse to join their club.”

Cruz and Rubio were born less than six months apart. Although their style could not be more different, there is little to separate them on policy. Cruz has rebranded himself as an isolationist and now says that mass-murdering Syrian dictator Bashar Al-Assad is the best hope of defeating Islamic State (IS). Rubio criticises Cruz for his role in passing the USA Freedom Act, which constrains the National Security Agency’s bulk collection of personal data, saying that this makes the country less safe.

As Rubio struggles to increase his support beyond the low double digits, New Jersey Governor Chris Christie has spotted an opening. He has been campaigning heavily in New Hampshire, the second state to vote, and in the latest surveys there he has crept up to 11%, level with Cruz and only two points behind Rubio.

Christie has sins of his own that Republicans will not easily forgive, having supported gun control and abortion in the past, and accepted Obamacare’s expansion of Medicaid health insurance for the poorest in his state.

The Rubio campaign indicated how seriously it takes his threat by running an advert showing him embracing Obama in 2012 when the President visited New Jersey to inspect the damage caused by Hurricane Sandy. Christie responded by calling Rubio a novice, telling talk radio host Laura Ingraham that in presidential debates Hillary Clinton would “pat him on the head and then cut his heart out.”

Bill Clinton is the only candidate, Republican or Democrat, to secure the nomination without winning in either Iowa or New Hampshire. Unless the polls are wrong, these are the historical odds that Rubio is trying to beat, by hanging on as the top candidate that the establishment can stomach, in the hope that when the primaries switch to more favourable territory, he will develop momentum.

To do that, he will have to get through a series of votes in the south, where Cruz is strong and the nativist wing of the party provides Trump with a large constituency. On March 15, Florida, Illinois and Ohio go to the polls. To maintain his challenge, Rubio will need to win at least two of them.

New Yorker columnist John Cassidy has suggested that parties need to suffer three defeats in a row before they will make the ideological and structural shifts required to be competitive again. Bill Clinton’s centrist, tough-on-crime, Wall Street-friendly Democrats emerged following three successive Republican victories. Tony Blair’s New Labour rose from the ashes of a third trouncing by Margaret Thatcher’s Conservatives.

Two sweeping midterm victories have given the Republican party control of both houses of Congress, but faced with the larger, younger, more diverse electorate that turns out on when the next president is on the ballot, it has only won the popular vote in one of the last six elections. Its choice of standard-bearer for this November’s contest will show what it has learned from those losses, and whether it is ready to break the cycle.