Democrats, who have pleaded for unity among their many factions for decades, finally achieved it Monday at the Democratic National Convention - at least for a night.

For example:- For the first time in as long as Credentials Committee Chairman Leroy Irvis said he can remember, no one challenged the credentials of even one delegate. Usually, competing factions argue bitterly that others robbed them of delegate seats.

- Unity was the reason Jesse Jackson decided Monday not to try to wrestle the vice presidential nomination from Lloyd Bentsen, the choice of apparent presidential nominee Michael Dukakis. Appreciative Dukakis and Jackson delegates honored that move all night with chants of "Jesse, Jesse."

- Unity was the theme of all major speeches Monday. Delegates and speakers from all the Democrats' major factions - labor, blacks, Hispanics, women and others - reveled in their common goal to regain the presidency by pulling together for Dukakis.

Democratic leaders hoped the unity would not crumble when Jackson forces attempt to add several planks to the party platform that are opposed by Dukakis - such as calls to increase taxes for the wealthy (Story on A-2.)

Former President Jimmy Carter best expressed the concern: "We are the party of diversity. Look around this hall - you'll see all the beautiful variety that makes

America strong and special and great. Because we are so inclusive, many of the conflicts of American life are fought - and creatively resolved - within our ranks.

"That is our strength. But it is also our danger. That is why, if I had to give you just one special message tonight, I would summarize it in a single word: Unity. One more time: Unity."

Carter gave one of many speeches during a night of emotion that whipped delegates into a frenzy.

To help his call for unity, Carter praised Jackson, saying he has "stirred the soul of our country."

But his greatest praise came for Dukakis.

He compared him to Woodrow Wilson because he is "idealistic," to Harry Truman because he's a "tough fighter," to Franklin D. Roosevelt because he's an innovator; and to John F. Kennedy because "he is from Massachusetts, he has chosen a running mate from Texas and he's going to defeat an incumbent vice president."

Keynote speaker Ann Richards, the Texas state treasurer, urged delegates to remember traditional Democratic values of strengthening the family and defending the common man - and did it in a way that had the delegate laughing and cheering . . . in unity.

"I'm delighted to be here . . . because after listening to George Bush all these years, I figured you needed to know what a real Texas accent sounds like," she said with a thick Texas twang.

"Twelve years ago, Barbara Jordan, another Texas woman made the keynote address to this convention, and two women in 160 years is about par for course. But if you give us a chance, we can perform. After all, Ginger Rogers did everything that Fred Astaire did. She just did it backwards in high heels."

About President Reagan, she said, "For eight straight years the leader of the greatest nation in the free world has pretended to us that he cannot hear our questions over the helicopter noise. . . . And when we get our questions asked . . . the only answer we get is, `I don't know,' or `I forgot.' But you wouldn't accept an answer like that from your children."

She said the Reagan administration had tried to impose rule of the wealthy over the common people by "dividing and conquering" into "selfish special interests."

But she told delegates: "This election is a contest between those who are satisfied with what they have - and those who know we can do better. . . . It's about the American dream. Those who want to keep it for the few - and those of us who know it must be nurtured and passed along."

Former Utah Gov. Scott Matheson, a friend of Richards, praised her speech as equaling or surpassing the much admired speech of keynoter Mario Cuomo, New York governor, four years ago.

"She unified, boosted and energized this convention, and gave it just what it needed," Matheson said.