Newt Gingrich had planned a mostly quiet weekend, a short break from the rigors of the campaign trail after a busy week. Instead, he was on the phone all Saturday morning, holding a tele-town hall with Iowans and later a conference call with reporters. He apparently realized he cannot afford to rest when his candidacy is on the line.

The call to Iowa voters highlighted the twin problems Gingrich now confronts in his bid to win the Republican nomination. One is answering his rivals’ attacks, which are raining down on the former House speaker in the form of television commercials and direct mail. Gingrich described the mail attacks as “junk and dishonest” and promised regular calls to rebut them.

The other problem is how to rapidly build a campaign infrastructure large and sturdy enough to sustain a viable presidential candidacy. Gingrich is months behind in the laborious process of identifying and mobilizing supporters in Iowa. Underscoring that fact were the repeated appeals for active support that he made to the 14,000 Iowans his campaign said were listening in on the call. Press 1 to speak for him at the caucuses on Jan. 3, he said. Press 2 to become a precinct captain.

The former speaker’s presidential campaign is now in its fourth and potentially decisive phase. The first stage was anticipation — the period from early this year until he finally entered the race for the Republican nomination. It was a time of stumbles and false starts.

The second was irrelevance, the long period that began with his attack on House Budget Committee Chairman Paul Ryan’s Medicare proposal and his Greek cruise vacation and continued after his campaign imploded. Though Gingrich was in the campaign, he was rarely in the conversation — more a historical figure commenting from the sidelines than a contender.

The third was rebirth, which encompassed his surprising reemergence as a force in the debates, his sudden rise in the polls and his customary self-confidence in declaring that, in all likelihood, he would be the Republican nominee. His road back has been visible in his movement on the stage at the many Republican debates, from the wings where the also-rans have been relegated to the center spot reserved for the leader in the polls.

The fourth phase — testing — is now underway. It began in the past two weeks when his rivals, deciding he might be for real, began to attack him and the party elites who fear the possible consequences of his nomination also launched an assault. Collectively, those groups have raised questions about his conservatism, his consistency, his reliability, his stability, his fitness. Gingrich maintains that he will run a positive campaign, but he knows the power of attack politics, having made it a principal tool of his rise to power in the House.

His unorthodox campaign is being tested, possibly beyond its limits. Conference calls or other tools of technology may help short-circuit the process of building an Iowa organization or answering attacks. But without the money and infrastructure of his principal rival, Mitt Romney, he is at a potentially enormous disadvantage as the campaign moves from the debate stages to the close-in competition in advance of the voting in the early states.

Romney is executing a campaign plan long in the making. Certainly he has been forced to adapt his tactics because of Gingrich’s recent successes. The best evidence of it came last week, when he used multiple interviews with the media to go after Gingrich. Those attacks have been amplified by the “super PAC” supporting his candidacy, which is pouring money into negative ads aimed at Gingrich that are running constantly in Iowa.

Romney is capitalizing on elite opposition to Gingrich. He is also trying to make himself at least acceptable to those in the Republican base who don’t trust him and who think he is not sufficiently conservative. The endorsement he received Friday from Gov. Nikki Haley of South Carolina highlights his efforts to both nail down the backing of elected Republican leaders and lure tea party conservatives by winning over some of the politicians with whom they most identify.

Gingrich is capable of building large enterprises. He did it in the 1980s and ’90s during his rise to power in the House. He did it as a private citizen when he left the speakership and the House. But what he’s trying to do now may be unprecedented, given time constraints and available money.

The attacks are clearly hurting. One that stings the most concerns the $1.6 million in consulting fees that he took from Freddie Mac. Michele Bachmann hit him hard on this in Thursday’s Fox News debate in Sioux City, Iowa. On Saturday, the Wall Street Journal editorial page hit him even harder, accusing him of a “lack of candor” in explaining what he did for the government-backed housing agency.

The editorial concluded: “Mr. Gingrich would help his candidacy if he stopped defending his Freddie payday, admitted his mistake, and promised to atone as President by shrinking Fannie and Freddie and ultimately putting them out of business.”

On Saturday, Gingrich told Iowans that most of the money “didn’t go directly to me” and instead went for overhead and expenses for his company, which provided consulting.

Also hurting him are questions about whether he’s a real conservative. Gingrich had a conservative voting record in the House. He has a history of helping to build the modern conservative movement. He was a conservative insurgent against the Republican establishment when he balked at signing onto the 1990 budget deal negotiated by then-President George H.W. Bush.

But as a party leader whose ultimate goal was to create a Republican majority, he applied his conservative principles pragmatically. House Speaker John Boehner put it well last week when he said at a breakfast sponsored by Politico, “It would be hard to describe Newt as not conservative,” before adding, “I am not sure he’s as conservative as some people think he is.”

At Thursday’s debate, Bachmann accused Gingrich of having supported Republican candidates who opposed a ban on late-term abortions. Gingrich’s response: “I don’t see how you are going to govern the country if you are going to run around and decide who you are going to purge.” It was the same philosophy that put Gingrich at odds with tea party activists in the special election in New York’s 23rd congressional district last year.

A loss in Iowa would be a serious blow to Gingrich’s candidacy, given where he stood two weeks ago, and he has two tough weeks ahead before the caucuses. He’s shown this year why he shouldn’t be underestimated. But he is also showing that he understands there is nothing at this moment that he should be taking for granted.

More coverage on Election 2012

Gingrich holds Iowa telephone town hall