THE Democratic candidates and the public take a welcome half-time break from the nomination campaign after this week's Pennsylvania primary. But before taking up the midpoint totals, it should be noted that the half-time show in Washington itself could unexpectedly bear on the second half of the Democratic contest, which resumes in a big way May 5 with the Texas caucuses. The Reagan administration has had to deal in recent days with congressional and world reaction to its CIA-guided mining of Nicaraguan harbors. The Soviet Union pounds away at its theme of Reagan administration intransigence , bringing up everything from arms control to the Olympics as evidence of soured relations. Elections in El Salvador, Iran's continued assault on Iraq, Congress's deliberations on the defense budget, the upward course of interest rates, the Meese affair - these issues already on the table argue caution in anticipating the context or outcome of the nomination battle before its June 5 climax in California, New Jersey, and West Virginia.
Still, Walter Mondale has to be feeling more confident and relieved. He has half the delegates he needs for the nomination. The sharp challenge by Gary Hart has made him earn, in tough on-the-road slogging, the delegates that the voters seemed unwilling to give him at the outset by acclamation. By now, the argument that his front-runner position was bestowed on him by ''special interests'' looks rather tattered. Teachers, unionists, and others in the Mondale coalition have themselves had a useful awakening.
The Hart challenge has also positioned Mondale as a more centrist candidate in foreign affairs. Mondale cast himself as the candidate of stability, with Hart arguing for a pullback in American commitment abroad and Reagan as the advocate of a more assertive, active US role.
To revive his campaign, Senator Hart must reestablish himself as a candidate of consistency in American responsibility abroad as well as of innovation in domestic economic and defense policy. He must bring older voters and lower income Democrats into his fold. The test of his resilience and resourcefulness against Mr. Mondale will reflect on his comeback powers as a leader.
The Rev. Jesse Jackson's campaign poses problems and potential of its own. Mr. Jackson, drawing few white votes, appears headed to the convention as a power broker for the Democratic Party's black wing. This is no small feat. But unless he disavows the threat of violence made on his behalf by Black Muslim leader Louis Farrakhan, he risks provoking serious racial polarization in the campaign. Already Farrakhan's denunciation of Jews, on top of Jackson's own ethnic slurs and Mideast positions, have given Republicans an opening to exploit a Jewish backlash vote. Adding unrebuked threats against the safety of black reporters could feed a white backlash in November. Such a prospect should be stopped now.
Meanwhile, observers and contenders alike have been surprised by the turns so far in the 1984 primaries. Especially given Washington's contentious mood, there is no reason to think fewer surprises lie ahead.