DES MOINES, JAN. 21 -- With 17 days left to go before Iowa's critical caucuses, a major reshuffling in the tight Democratic presidential field has moved Rep. Richard A. Gephardt (Mo.) into a shaky lead and abruptly forced Sen. Paul Simon (Ill.) to attack.

Former Colorado senator Gary Hart, meanwhile, is rapidly losing strength and is no longer considered a serious contender in this state. A poll taken for one candidate suggests Hart could finish as low as sixth among the seven Democratic candidates.

Gephardt's upward movement was confirmed today in two polls. An NBC-TV poll, taken Sunday to Tuesday, showed Gephardt supported by 19 percent of those who say they are likely to attend the Iowa caucuses, Massachusetts Gov. Michael S. Dukakis by 18 percent, Simon by 15 percent, Hart by 14 percent, Jesse L. Jackson by 8 percent, former Arizona governor Bruce Babbitt by 8 percent and 15 percent undecided. The opinions of 720 Iowa Democrats were polled.

Meanwhile, a Los Angeles Times poll gave Gephardt 23 percent, Simon, 21; Dukakis, 15; Hart, 12; Babbitt, 11, and Jackson, 8. Margins of errors in the polls essentially mean there is a dead heat at the top.

Private polls taken for candidates during the last week have reached somewhat conflicting findings. Simon pollster Paul Maslin, for example, said his surveys continue to show Simon narrowly ahead of Gephardt.

But there is general agreement here that Gephardt has steadily gained strength in recent weeks, while Simon has slipped and Dukakis stalled.

Babbitt, until recently the darkest of the dark horses in the field, is the only other candidate gaining strength. But he barely makes it into double digits in any poll.

"It's not out of the question that Babbitt could finish fourth or even third, but it looks like he has very limited growth potential," said one campaign pollster. "He has absolutely no appeal outside of Des Moines or Cedar Rapids."

The race here remains extremely fluid, if not wide open. But the lineup has changed sharply in the last month.

At that time, Simon was the clear front-runner and Gephardt was limping toward the end of a frustrating year of campaigning that had seen his fortunes rise and fall. At one point, Simon had been as high as 29 percent in the Des Moines Register's Iowa Poll and Gephardt as low as 6 percent.

But Hart's reentry into the Democratic contest last month halted Simon's upward momentum and confused the race.

Things did not begin to sort out until Hart gave an uninspiring performance during his first debate appearance with his six rivals last Friday night. The Hart mystique seemed to evaparate for many voters.

Gephardt began a slow, steady climb in the polls after the Christmas holidays with a crisper message and a $300,000 television advertising campaign. The ads were based on populist themes and pictured Gephardt as a "fighter."

The latest poll numbers seem to have thrown a scare into Simon, who has based his campaign strategy on a win in the Feb. 8 caucuses. He is being squeezed by Dukakis and Babbitt for the liberal urban vote and by Gephardt for the rural and blue-collar vote.

He reacted dramatically this week. He abruptly canceled a New England campaign swing Friday to spend more time in Iowa and launched an attack on inconsistencies in Gephardt's voting record.

Simon appears uncomfortable on the attack. In news conferences Wednesday and today, Simon tried to contrast his record as a "consistent" defender of "the heritage of the Democratic Party" with other candidates who "have bought into the Republican mindset" and refused to stand up "against the skewed Republican priorities of the past seven years."

Simon today refused to say whether Gephardt is part of the crowd he is attacking. "I'm saying I have stood up very clearly," he said at a news conference in the rotunda of Iowa's capitol. "You can draw your own conclusions."

Simon said Gephardt supported Reagan-backed tax cuts in 1981 and 1986 that "caused huge budget and trade deficits."

"I opposed the grain embarago" against the Soviet Union, Simon said. "He supported it. I opposed the 1985 farm bill along with {Iowa Sens.} Tom Harkin and Chuck Grassley. He supported it. Yes, there are differences. No question about it," Simon added.

Asked about the attacks, Gephardt said, "I think in some ways Sen. Simon is an ideologue."

Simon retorted: "I don't think I am an ideologue. . . . I think I am a very practical politician."

Dukakis has also done some recent retooling, trying to appear more aggressive and compassionate. As part of this effort, the normally mild-mannered Dukakis aggressively attacked Babbitt during a debate Wednesday night and his campaign began airing ads stressing Dukakis' opposition to aid to the Nicaraguan contras. He also brought Sen. Edward M. Kennedy (D-Mass.) to the state to campaign for him today.