As of this week, it’s still 16 months to the next US presidential election, and seven months to the Iowa caucuses, the first event that will count towards choosing the candidates. But the campaign for the Republican nomination is already well under way, with news this morning that front-runner Mitt Romney has raised $18.25 million in the last quarter, well ahead of his rivals.

Presidential campaigns are an expensive business, even in the early stages when serious candidates are trying to establish nationwide organisations. But no one really needs that much money; the substance of Romney’s campaign would not be significantly affected if he had only raised half as much. (Particularly since he can always spend his own money — last time he ran it cost him $44.5 million.)

The real importance of the fund-raising numbers at this stage is as a proxy for popular support. Since no actual voting takes place for so long, pundits look at money as a substitute, figuring that if people aren’t donating to a candidate then they’ll probably also be less likely to vote for them when the time comes. So dollars drive expectations, which in turn drive further media coverage.

On that basis, Romney’s achievement is good but not stunning. He’s actually raised about $3 million less than he did in the corresponding period four years ago. But his advantage over the other declared candidates is much bigger than it was then. No one else raised more than $5 million in the past three months — with the notable exception of Michele Bachmann, whose campaign has yet to report its total (the deadline is next week).

If no further high-profile candidates enter the field — Rick Perry and Sarah Palin being the obvious possibilities — then the GOP race looks like coming down to a straight fight between Romney and Bachmann, in which Romney would be heavily favored.

Tim Pawlenty, who raised $4.2 million, is still viable but has failed to make an impression in the polls in Iowa. Jon Huntsman and Herman Cain remain very much long shots, while Newt Gingrich — whose $2 million in fundraising was insufficient to pay off his debts — has already been relegated to the also-rans.

A Romney-Bachmann contest would be bad news for the New Republic’s Jon Chait, who for some months has been running a “Mitt Romney death watch” — with the theme that Romney’s moderate positions made him unacceptable to the GOP electorate. Last month, with Romney still riding high in the polls, he upgraded his status to “mostly dead.”

Today Chait is saying that while “a one-on-one match-up between Romney and Bachmann is Romney’s dream scenario … it’s also Bachmann’s dream scenario — she gets to face off against an establishment candidate totally unacceptable to large segments of the party base.”

That’s what gives scope for a new candidate, most likely Perry, to split the difference between mainstream Romney and crazy extremist Bachmann.

But as I pointed out a few weeks ago, that sort of opportunity has a downside — a compromise candidate is subject to attacks from both sides, and may end up satisfying nobody.

It may that Perry would beat either Romney or Bachmann if the race pitted him against them one-on-one. But to get to that point he first has to get into the top two, and that will be difficult. And the longer he leaves it, the more difficult it looks like becoming; candidates who declare this late in the piece have little chance unless they start with very high name recognition.

The one potential runner who has that, of course, is Palin, but her problem is that Bachmann has now occupied so much of her natural territory that she would find it hard to get a campaign going — and she is not the sort of person who’s likely to make the effort if she looks to be facing ignominious defeat.

That’s not unequivocally good news for Romney, since it might mean his opponents rally all the more quickly behind a single candidate such as Bachmann. But since the GOP establishment will pull out all the stops to avoid nominating her, Romney might reasonably think he has her measure.

As of this morning, Intrade is still giving odds of almost 2/1 against Romney for the nomination. That looks like pretty good value to me.