Chief correspondent

— Darkness was beginning to envelop the city of Chicago late Friday afternoon, but at President Obama’s reelection headquarters the vast open room that houses the campaign staff was still packed with workers.

Jim Messina, the campaign manager, said there would be no New Year’s weekend passes for his army.

“Iowa is Tuesday,” he said.

Messina was referring to Iowa’s precinct caucuses, but not those drawing all the media attention — the ones that will start the process of selecting a Republican presidential nominee. Democrats, too, will be caucusing Tuesday night, and the Obama campaign views the gatherings as one more opportunity for organizing in a state they expect to be among the many that will be fiercely contested in November.

On Tuesday night, as Republicans fill in their ballots, the president will address Democratic caucus attendees by video link from Washington. His presence signals the Obama campaign’s determination not to be drowned out by the GOP race and to use whatever platform is available to cast Republicans as protectors of the privileged at the expense of the middle class.

As Republicans squabble over their pick, the Obama team is already well deployed in Iowa and elsewhere. Messina said his paid staff there numbers “north of 20,” arrayed in eight field offices across the state. Until the Republican presidential campaigns finally ramped up in December, the Obama campaign may have had more paid staff members on the ground in Iowa than all the Republican campaigns put together.

The Obama team has been preparing all year for what it anticipates will be a much more competitive general election than in 2008. While campaign officials believe the president is starting 2012 in a slightly better position than he was in last summer, they also recall that even in what was considered a relatively easy victory four years ago, 47 percent of the electorate voted against him.

Messina, having seen conventional wisdom about the GOP race change by the month, said he can’t pretend to predict whether the other party’s nomination contest will be over quickly or drag on for months, as Obama’s race against Hillary Rodham Clinton did in 2008.

“Our goal was by the end of this year to be ready for whenever it ended,” he said. “We’re ready now.”

The campaign has organizations up and running in all the battleground states. How many?

“We don’t talk about that,” he said, although with the campaign’s various possible routes to 270 electoral votes already well publicized, it is not difficult to compile that list.

Nor will Messina talk about the technological bells and whistles that have been undergoing testing in a few states and will soon be rolled out across the country. What he will say is: “We’ve always believed that 2011 was about building the strongest grass-roots campaign in modern history, and we’re on track to do that. . . . I believe we’re going to be better, faster and more focused on the ground.”

Since it opened the national reelection campaign office overlooking Chicago’s Millennium Park this past spring, Obama’s team has made more than 350,000 calls to supporters in Iowa, according to an official. Workers have held 4,000 one-on-one conversations with voters, a signature of the grass-roots style that has long been part of the Obama campaign’s DNA. And they have held 1,200 training sessions, house parties or phone banking meetings.

In other states with early primaries or caucuses that are also likely to be competitive in November, the intensity is nearly as great as it has been in Iowa. In Nevada, where the foreclosure crisis has led to one of the nation’s highest unemployment rates, in turn posing a threat to the president’s reelection, the campaign has made nearly 500,000 phone calls.

Obama’s campaign advisers also won’t predict the outcome of the Republican race, though they have tipped their hand repeatedly by never taking their eyes off former Massachusetts governor Mitt Romney.

If they believe Romney is still Obama’s likeliest opponent, however, they do not rule out more surprises before the Republican race has run its course, although the absence of a well-funded and credible challenger gives Romney a clear advantage almost regardless of the outcome in Iowa on Tuesday.

Obama advisers see Romney as both formidable and potentially flawed, a candidate who has moved to the right in pursuit of the nomination and a former businessman whose background they will attempt to exploit in the debate over fixing the economy.

“I think we’d be foolish to treat him or frankly any opponent as anything but formidable,” said David Axelrod, the Obama campaign’s chief strategist. “But do I think he is an ideal candidate? No, from their standpoint. I think there are deep problems there. In some ways, [he] offers the purest debate over what direction we want to take as a country.”

Obama officials find it amusing that Romney has tried to distance himself from the millions of dollars in negative ads the super PAC supporting his candidacy has aired, to devastating effect, against former House speaker Newt Gingrich.

“He was in trouble, and he signaled for the Air Force, and they came in with shock and awe,” Axelrod said.

The president’s year-end defeat of House Republicans over the extension of the payroll tax cut has brightened the mood in the Obama camp. That victory ratified the strategic shift that took place just after Labor Day.

Officials say the turning point in their thinking came late this summer, after the debt-ceiling negotiations that ended with no significant agreement and political damage to both Republicans and the president.

The president decided to stop negotiating with Republicans in Congress and to take his case to the people. His September jobs speech and his later speech in Kansas about middle-class values sharpened and encapsulated the message that will be the foundation of his reelection campaign.

His coming State of the Union address will add “tangible ideas” to the broad goals he has been articulating, advisers said. On the campaign trail, he and his team will continue to pound congressional Republicans and the party’s presidential nominee as out of touch with the lives of ordinary Americans and offering stale policies to boost the economy.

Romney is already accusing Obama of running an almost totally negative campaign, but few doubt that both sides will rely heavily on attacks. The animosity between the parties that has been building has set the tone for the election year. The existence of well-funded super PACs virtually assures a high volume of negative ads, and neither Obama’s team nor those of the GOP hopefuls show any hesitancy about going after one another.

Romney has spent the past two weeks admonishing Gingrich for complaining about being the victim of so many attacks, which the Obama campaign team takes as a sign of Romney’s willingness to mix it up in the general election. They also note how aggressively Romney’s operation went after Texas Gov. Rick Perry, and then Gingrich, when each posed a threat.

Beyond the negativity, however, the general campaign is likely to feature a great debate between the parties, a clash of philosophies rooted in distinctly different views of the economy and how to bring prosperity to the greatest number of people. In that sense, the election that will begin to unfold this week may be more significant than 2008’s.

As Messina put it, “This country is exactly at a crossroads, and it feels it.”