Concord, N.H. — When New Hampshire citizens vote today, they will set the tone for Election ' 84. Walter Mondale remains the front-runner. But if he is upset, or if another candidate, such as Gary Hart or John Glenn, runs close to him, it could change the character of the race that Democrats run against Ronald Reagan in the fall.
Within the Democratic Party today, there are at least two major factions tugging at each other, vying for influence in the nation's largest political organization.
Mr. Mondale - in what some experts consider brilliant strategy - has pulled together much of the old Democratic political alliance of Franklin D. Roosevelt. This includes labor, blacks, city dwellers, and the South.
The former vice-president's strength cuts across all of the party's ideologies - conservative, moderate, and liberal. He has run a campaign without major mistakes, without alienating any important group that could help him.
Yet there are Democrats who worry deeply about their party's future, and who argue that Mr. Mondale and other leading Democrats who have endorsed him, such as House Speaker Thomas P. O'Neill Jr., are sending the party toward another defeat in 1984.
These other Democrats have been sympathetic to the arguments of such candidates as Sens. Ernest Hollings, Gary Hart, and John Glenn. Senator Hollings , who has watched his native South go Republican over and over again since the days of Dwight Eisenhower, says that the party has an image problem with mainstream Americans.
Walter Mondale, according to Mr. Hollings and others, fails to address that image problem effectively. The problem involves two areas - the economy and national defense - and these have become the main debating points of this Democratic campaign.
To win in November, Senators Hollings and Glenn contend, the party must do two things:
* It must clearly stand for policies that show that Democrats are not the party of ''big spenders'' who will bring back high inflation and high interest rates to the American economy.
* It must demonstrate that the party believes in a strong defense posture, which poll after poll shows is supported by the American people.
Therein lies the problem, both for Mondale and his fellow candidates. Democratic Party politics is dominated by activists, by liberals, and by big labor. It was these who cast the deciding votes in last week's Iowa caucuses. It is these who do much of the work, who contribute, and who control the organizational clout that lines up behind Democratic candidates. And it is these who fight hardest against stronger defenses and lower domestic budgets.
Yet some Democrats argue that the candidate most popular with party regulars isn't necessarily the candidate with the best chance of winning in November.
This is one reason that some party officials in the South, for example, speak privately in worried tones about Mondale's strong and open support from labor unions. It may help him now in Iowa and New Hampshire. But in the fall it could be a burden that the party cannot carry in Sunbelt states, where big labor remains unpopular.
This intraparty struggle is almost certain to continue, even if some candidates such as Hollings and Glenn fail to reach their goals in today's voting.
It may take a different form. There has been growing support, for example, for Gary Hart's call for ''new ideas'' and ''new leadership'' in the economy (where he has proposed new policies to meet competition from abroad) and in defense (where he has called for a stronger military through more effective weapons systems, rather than sharply higher spending).