Rep. Richard A. Gephardt (D-Mo.), turning his presidential campaign almost entirely into a drive to win the Iowa caucuses, has shifted the bulk of his staff to that state in the first clear reaction to Gary Hart's reentry into the Democratic contest.

Gephardt's campaign manager, William Carrick, said yesterday that some 40 national campaign staff members now based in Washington will be reassigned to field work in Iowa, leaving about 15 here. Joining them will be key political operatives from the Missouri, North Carolina, Texas and Georgia campaign staffs.

Gephardt's all-or-nothing drive in Iowa is based on the calculation not that Hart will win there, but that only the Democratic winner in that state will be taken seriously in the next state, New Hampshire. Under this theory, whichever candidate wins Iowa -- but not the second or third place finishers -- will get a big boost in New Hampshire enabling him to compete against the two candidates expected to be the strongest there, Massachusetts Gov. Michael S. Dukakis and Hart.

"Hart's reentry into the race takes votes {in Iowa} from Dukakis and Simon," Carrick said, comparing an NBC poll taken before Hart's decision and a recent poll taken in Iowa for WBZ television in Boston, which put Hart at 22 percent, Illinois Sen. Paul Simon 17 percent and showed Dukakis and Gephardt each with 14 percent. The NBC poll taken before Hart rejoined the race showed Simon leading with 21 percent, followed by Dukakis with 20 percent and Gephardt with 10 percent.

A new Des Moines Register poll, scheduled to be released today, gives some credence to the theory that Simon is hurt in Iowa by former senator Hart's reentry. The poll finds Hart with an 11-point lead over Simon.

Simon's Iowa campaign manager Pat Mitchell said Hart will not cause them to reshuffle their organizational deck but that they, too, will "greatly enlarge" the staff in January. He said, "We've been adding people over the course of the last three weeks at a steady pace. It will pick up significantly after the first of the year. We're up to 55 right now."

Carrick called the new playing field "an enormous opportunity for us and we want to take maximum advantage of it. New Hampshire will be a Hart-Dukakis race with room for the person who wins Iowa and we want to be that person."

Carrick said the Iowa effort will bring 25 to 30 members of Congress to the state in January to "adopt a county" and campaign for Gephardt. The weekend before the caucuses 50 to 60 House members will scour the state, he said.

As for money, Carrick predicted a "$1 million advantage over Simon" after matching funds are distributed. "We intend to fully exploit it," Carrick said. He said that the campaign "backloaded the expenditure of money during December. We didn't do TV advertising or direct mail, saving the budget for January."

Gephardt, who has been working in Iowa the longest of any of the Democrats, has been widely considered to have lost support there, as well as nationally. A Washington Post-ABC poll taken the past week of 318 voters who are Democratic or leaning that way showed Gephardt sliding from being the choice of 10 percent to 2 percent. One Democratic adviser to another campaign noted, "He hit the pavement and splattered. He has got to resurrect. He's turning his national campaign into an Iowa campaign because that is the only hope."

Tad Devine, Dukakis field director, said Gephardt "is drawing a line in the sand -- it's the borders of Iowa. Now it's officially make or break for Gephardt in Iowa."

Most of the other Democratic campaigns -- and the Republican ones as well -- had already planned big bursts of efforts in Iowa after the first of the year simply because of timing -- its caucuses are the first, Feb. 8. The campaign of former Arizona governor Bruce Babbitt, among the poorest of the group, plans to double its staff, from 30 to 60 paid workers, and to spend at least $300,000, plans that are unaffected by the Hart move. Gephardt's beefing up will double his Iowa staff to about 100.

Devine said there will be an expansion, but not reallotment, of people into Iowa and New Hampshire by the Dukakis campaign.