Analysis-Trump is on track to win in Iowa - but can he deliver a knockout punch?

By Nathan Layne and Tim Reid

DES MOINES, Iowa (Reuters) - Donald Trump appears headed to victory in Iowa, but the size of the win matters if the former U.S. president wants to deliver a knockout blow to key rivals heading into Republican Party nominating contests in other states.

With three days to go until the party's first contest in Iowa, opinion polls indicate former United Nations Ambassador Nikki Haley is closing the gap with Trump in New Hampshire, which holds its primary on Jan. 23. Florida Governor Ron DeSantis has slipped in New Hampshire polls, but is banking on a breakout performance in Iowa to revive his campaign.

Trump heads into the Iowa caucuses on Monday with outsized expectations. Political website FiveThirtyEight, which compiles an average of public opinion polls, pegs Trump's support in Iowa at 52 percent, more than 30 percentage points above Haley or DeSantis.

Trump campaign advisers have sought to temper expectations by noting that the previous record win in a contested Republican caucus was Bob Dole's 12.8-point margin in 1988.

But four political analysts interviewed by Reuters said Trump needs a more convincing victory closer to the 30 point-plus margin suggested by the polls to blunt Haley's momentum.

Doug Heye, a Republican strategist, said Trump needs to win by at least 15 to 20 percentage points or risk losing the air of inevitability, a situation that could allow Haley or DeSantis to pick up momentum at a critical juncture in the race.

"That's a bare minimum for Trump. Anything below that shows - and will get blown up as - vulnerability," Heye said.

Chris LaCivita, co-manager of the Trump campaign, said he was confident that the "intensity" of the former president's base would translate into a big win despite the cold gripping the Midwestern state. But he stopped short of predicting the kind of win suggested by the polls.

"A win's a win, but no one has ever won by more than... 12.8," LaCivita told reporters on Thursday.

One wild card is turnout. Reflecting a worry that his supporters will stay home given his comfortable polling lead, Trump has warned against complacency at recent rallies.

Another risk for Trump is a strong showing by Haley, who has been rising in the polls. If she were to secure second place, it could establish her as the clear alternative to Trump, giving her a boost in New Hampshire, analysts said.

Haley campaign spokesperson Olivia Perez-Cubas would not quantify how close they hoped to come to Trump on Monday. "We will have a strong showing in Iowa and ride that momentum into New Hampshire," she said.

Two polls released this week showed Haley cutting into Trump's advantage in the New England state. A CNN poll conducted with the University of New Hampshire placed Trump's lead at just 7 percentage points, while a separate survey by USA Today/Boston Globe/Suffolk University put him ahead by 20 points.

Rather than Dole's 1988 victory, Iowa State University professor David Peterson views the 1980 Democratic caucus, when then-President Jimmy Carter beat challenger Senator Ted Kennedy by nearly 30 points as a better benchmark. Viewing Trump similarly to an incumbent, Peterson is expecting a 35-point win.

"Given the strength of Christian conservatives in this state and his position in the party, 20 points would be underwhelming," Peterson said. "Anything smaller than that I don't think is a knockout punch."

POOR PREDICTOR

To be sure, Iowa has a mixed track record of predicting success in later states. In the eight contested caucuses between 1976 and 2016, only three Republican candidates who won Iowa became the party's eventual nominee.

In contrast to 2016, when Trump was out-organized in Iowa and lost narrowly to Senator Ted Cruz, the former president's campaign has built an expansive data mining and get-out-the-vote operation here.

No candidate has staked more on a strong result in Iowa than DeSantis: He visited all 99 counties in the state, fiercely courted its socially conservative voters and secured its governor's backing.

DeSantis associates acknowledge that a third-place finish in Iowa would likely doom his bid, given that he is polling well behind Haley in New Hampshire and would struggle to beat her in her home state of South Carolina's primary on Feb. 24.

"Ron DeSantis is only focused on outworking and out-organizing the competition. We believe that Iowans will reward our approach," said Andrew Romeo, communications director for DeSantis.

Iowa's 40 delegates to the Republican National Convention will be awarded on a proportional basis, compiled from the roughly 1,700 precincts set up in local schools, churches and community centers across the state.

Even if Haley performs well in Iowa and goes on to win New Hampshire, Trump should still be considered the favorite to win the nomination, said Kyle Kondik, an elections analyst at the University of Virginia Center for Politics, noting that Trump is still far ahead of Haley in South Carolina and other key states.

(Reporting by Nathan Layne and Tim Reid in Des Moines; additional reporting by Alexandra Ulmer in San Francisco; editing by Ross Colvin and Jonathan Oatis)