By Kevin Connolly
BBC News, Washington
We have already been reminded during this campaign of the world-weary political slogan that you campaign in poetry but you govern in prose.
The mood of America's Republicans as the primary season approaches its climax - with more than 20 states voting on Super Tuesday - could be paraphrased by a subtle variation on the theme.
You might rule as a pragmatist, but you have to run as a purist.
In other words, any Republican president is going to have to cut deals and make compromises with those pesky liberals in Washington once he's in the White House, but to get there he has to satisfy the party's conservative base that his heart and his principles are in the right place.
The race this year is complicated by the fact that none of the Republican candidates satisfies the party's core voters on all the three issues which really matter to them - strong national defence, small government with minimal taxation to encourage business, and conservative - preferably Christian - social values on issues like abortion.
The last candidate who scored a perfect 10 on all three was probably Ronald Reagan, who is revered as a kind of patron saint of the American right.
It says much about the state of the party that if he were running in 2008, he would win by a landslide.
Invoking memories of Reagan, and implying that you are the candidate who has most in common with him, are more or less compulsory features of campaigning this time around, 40 years after the great man first tried (unsuccessfully) to secure the party's nomination.
The candidates are keen to draw on Ronald Reagan's legacy
John McCain - whose victories in the primaries in Florida and South Carolina make him a front-runner on the way into Super Tuesday - is a clear winner of the Ronald Reagan Posthumous Seal of Approval Award with a TV ad that includes the line: "As a prisoner of war, John McCain was inspired by Ronald Reagan."
If he could have squeezed in a reference to God somewhere, he'd have composed the perfect Republican campaigning sentence.
Mr McCain's advertising also includes a precious clip of video of the two men walking together, and the Arizona senator often describes himself as "a foot soldier in the Reagan revolution".
Tapping into the Reagan legacy is Mr McCain's tactic for dispelling the doubts of Republican true believers who feel that he is simply out of step with grassroots conservative opinion on issues like illegal immigration, abortion and tax.
Before he began his latest run for the White House, that didn't matter much to Mr McCain.
He had a secure personal political base in Arizona and he enjoyed a certain popularity among independents, and even some Democrats, as much for his maverick opinions as for the personal courage he displayed as a Navy pilot and as a prisoner of war in Vietnam.
Now, of course, it matters hugely, and Mr McCain is wooing the Republican base.
Here is one small example. Senator McCain, the Washington old hand, has worked with Democrats like Ted Kennedy on legislation to regularise the legal status of the millions of workers who entered this country illegally; Candidate McCain talks of sealing the border with Mexico.
On the way into Super Tuesday, the Arizona senator enjoys a comfortable lead over his only serious rival for the Republican nomination, Mitt Romney, whose efforts to stir up conservative doubts over Candidate McCain are hampered by his own record in office - Mr Romney served a single term as governor of Massachusetts, which ended just over a year ago.
He came into office facing a budget deficit and was forced to increase state revenues to deal with it - his opponents say he increased taxes, he prefers the less politically loaded formulation that he increased "fees" (he put up charges for things like drivers' licences).
Also on his watch, civil unions became possible for same sex couples in Massachusetts, and the state enacted legislation which required nearly all citizens to buy health insurance or face penalties - more the kind of thing, say conservatives sniffily, that you'd expect from a liberal.
It is a tough situation for Mr Romney, who had to find a way as governor to work with a more liberal legislature.
If he says he personally disagrees with the sorts of things that happened in Massachusetts, he is accused of flip-flopping. If he doesn't, he is stained with deadly taint of pragmatism.
So for lots of Republicans, this race is about finding the candidate with whom they will feel least uncomfortable, and it looks as though Mr McCain is going to win.
He is aided by the fact that, where Democratic primaries work by proportional representation (you get some delegates even if you don't come first), Republican races tend to be winner take all.
Where Mr McCain has narrowly defeated his nearest challenger, as in Florida, he has scooped all the delegates.
He might well secure the nomination on Super Tuesday even though Mr Romney, a private equity capitalist with a personal worth estimated at a quarter of billion dollars, has spent a fortune on trying to remain in the race.
For a long time, the Republican race this year was a bit of a mess and Mr McCain only really emerged as front-runner after his wins in South Carolina and Florida.
Mr Giuliani quit the race and pledged to campaign for Mr McCain
It was to his advantage that the other candidates turned out to be a curious bunch.
Rudy Giuliani pressed the self-destruct button, attempting to re-write the rules of campaigning by focusing on Florida and ignoring the earlier-voting states. Look out for him in the "How Not to Do It" section of future text books of political science.
Fred Thompson, the actor and former senator from Tennessee, was an enigma. He looked the part, and said some of the right kind of things but was curiously lacking in energy - "tough-talking, but lazy" as someone memorably put it.
Mike Huckabee, the Baptist minister who is a former governor of Arkansas, is charming and amusing, but offers ideas - like the abolition of income tax - which do not seem thoroughly thought through.
Mike Huckabee has charm but his policies may not win over everyone
Texas Congressman Ron Paul asks good questions - like why is American effectively borrowing money from China to intervene in Pakistan - but was always likely to remain a fringe figure in the presidential race.
So the Republican party will most likely end up with John McCain as candidate in spite of the ideological doubts about him, and the awkward questions that Democrats will surely pose about his age (at 72 he'd be the oldest person ever to enter the White House) and his health (he is a cancer survivor).
And if he does emerge as the party's nominee - perhaps even as early as the middle of this week - he will face two key challenges.
Eventually, of course he will have to prepare to run against either Hillary Clinton or Barack Obama for the presidency itself - but first, he will have to unify his own party, satisfy its many doubters and persuade the true believers that he is worth working for, and worth voting for when the moment comes.