As the Democrats came together here this week, a Republican Party observer in the gallery at Madison Square Garden looked down on the milling crowd of delegates and mused: "It isn't going to be easy. Ronald Reagan can win this time, he really can, but it isn't going to be easy."

Even before defeated Democratic candidate Edward M. Kennedy had vowed to support Carter, the view from the Reagan camp was that the Republican nominee faced a tight, tough race against an incumbent who knows how to use the power of the presidency.

Reagan, who always runs scared, has trotted out his old gag about how "President Dewey has told me not to get overconfident."

Chief of staff Ed Meese has in recent weeks put Reagan's chances of defeating Carter at no better than 50-50 and maybe less than that.

In part, such cautious estimates are poor-mouthing intended to guard against overconfidence induced by national polls that show Reagan running away from Carter. In part, they may also reflect a prevailing view, reinforced by Gerald R. Ford's campaign in 1976, which nearly overcame a 30-point deficit, that any president is 10 feet tall.

But beyond these considerations, there is a genuine estimate on the Reagan side that the former California governor faces a difficult battle in seeking to force a new coalition of disaffected working-class Democrats, orthodox Republicans and independent ticket-splitters who some polls show are skeptical of Carter and of Reagan.

Despite the emphasis on unity here this week, the Reagan strategists believe there is a feeling among independent and swing voters that neither the Democratic Congress, which Kennedy represents, nor the Carter administration have the remedies for a lagging economy and a diminished defense.

This skepticism is likely to continue, even to grow, during the coming months. But Reagan strategists frankly acknowledge that the skepticism extends to the challenger as well -- that there are many voters who doubt whether anyone is up to the requirements of the U.S. presidency, and other anti-Carter voters who doubt whether Reagan is the answer.

One of the curious problems confronting Reagan is that voters who four years ago were "anti-Washington" have become distrustful of the ability of an anti-Washington outsider -- which Carter was in 1976 and Reagan is now -- to perform competently as president.

This gives Carter the ironic opportunity of suggesting that Reagan would be a poor president because he would be like the Carter of four years ago. And it makes it necessary for Reagan to mute some of his favorite antigovernment rhetoric and focus on his eight-year management of California state government.

Reagan will try to keep Carter on the defensive throughout the campaign. His strategists hope to turn aside the barrage of attacks frfom the president that began at the Democratic convention by suggesting that Carter is "smearing" Reagan because the administration's "failed presidency" won't stand public scrutiny.

"He [Carter] can't campaign on his record," Reagan campaign manager William J. Casey said Thursday on ABC News. His record is a record of failure in the White House and leadership in the presidency . . . So he probably has to resort to a campaign of personal abuse."

"What it boils down to" said another Reagan aide, "is whether we can keep Carter the issue. If Carter is the issue, we're going to win this election. cIf not, we're in trouble. It's as simple as that."