The contest for the Democratic nomination in the election to succeed retiring, 30-year veteran Rep. L.H. Fountain has come down to the question of whether more whites or more blacks can be motivated to get out to vote in today's runoff between H.M. (Mickey) Michaux Jr. and I.T. (Tim) Valentine Jr.

Michaux, a Durham attorney, former state legislator and U.S. attorney in the Carter administration, is seeking to become North Carolina's first black congressman in 80 years. His campaign depends heavily on a substantial turnout of black voters, who represent 36 percent of the registered Democrats in the 2nd Congressional District.

Valentine, a Nashville, N.C., attorney and a former state Democratic Party chairman, is encouraging whites, especially in the small cities and rural regions on the eastern side of the district, to go to the polls.

Both candidates have exercised self-control in the post-civil rights South. Although race is a major factor, the two men have not engaged in blatantly racial politics.

In a series of campaign encounters, the candidates have attempted to draw distinctions between themselves on such issues as a proposed constitutional amendment to require a balanced federal budget.

Valentine says he favors the amendment and proposes systematic reductions "in every single program." Michaux says he supports a balanced budget through elimination of "marginal programs," but he calls the proposed amendment a "smoke screen."

While they have been debating, the crucial work of their campaigns has been taking place at the telephone banks that each organization has set up.

The get-out-the-vote efforts have been hampered not only by the lethargy of midsummer and the beginning of the tobacco harvest but also by a shortage of campaign funds. Neither candidate was able to afford television advertising before the final weekend of the runoff campaign.

A turnout as low as 20 percent of the registered Democrats is anticipated, so that each candidate's chances hinge on the success of their organizations in motivating voters.

Michaux has received assistance from national black groups, organized labor and some veterans of the Carter campaign. Valentine has picked up support from Fountain's political backers.

Fountain has represented this largely rural district for 30 years, building a record as one of the staunchest conservative Democratic southerners to become a regular supporter of President Reagan's budget and economic policies.

He decided not to seek reelection after the state legislature added urban Durham County to the district.

The major emphasis of Michaux's campaign is assailing Reaganomics. He calls it "radical economics" and terms the 1981 Reagan tax cut "unwise and unfair."

However, Michaux also says he backs Reagan's hard-line policy toward the Soviet Union.

The Republican nomination was settled in the first primary on June 29 when Jack Marin, a Durham attorney and former professional basketball player, won.

Marin is running with the backing of the National Congressional Club, the organization of political supporters of Sen. Jesse Helms (R-N.C.).