MADISON, Wisc., Thursday, March 28--The specter of the "cross-over" is haunting Johnson supporters in Wisconsin.

Unlike the other 49 states, Wisconsin has an open primary. Voters who normally cast their ballots for one party can nevertheless cross-over and vote in the other party's primary simply by telling officials at the poll that they belong to that party.

President Johnson and Senator Eugene J. McCarthy are running neck-to-neck in Wisconsin. One recent poll showed them with 40 per cent each, and a 16 per cent write-in for Sen. Robert F. Kennedy '48 (D-N.Y.). If enough Republicans vote in the Democratic primary--either from a dislike of Nixon or boredom with the GOP's non-contest--they can swing the Democratic race.

Virtually all of this Republican cross-over will go to McCarthy, observers agree, since he offers the only clear alternative to Nixon. No one is willing to predict the exact amount of cross-over votes McCarthy will get, but it will be substantial--perhaps enough to beat Johnson. "It can do it," Leslie Aspin, executive director of Wisconsin citizens for Johnson-Humphrey, said recently.

McCarthy supporters say they have not organized any "Republicans for McCarthy" group here. They are counting on their canvassing of voters to reach Republicans as well as Democrats.

Denounced

Last Monday night, Mrs. Warren P. Knowles, wife of the state's Republican Governor, attended a McCarthy rally in Madison. Postmaster General Lawrence F. O'Brien denounced her for doing this, and said he "understood" that she was "busily engaged" in promoting a cross-over. Mrs. Knowles was not available for comment.

Most of the Republican cross-over will come from the southeastern part of the state, which includes the Milwaukee and Madison regions. Most of Wisconsin's liberal Republicans live there. Some cross-over in rural Republican areas is possible, but less likely. Wisconsin's conservative rural Republicans are more likely to stick with Nixon.

Officials of both regular party organizations, who hate the unpredictable open primary, have strongly urged party members to cross-over. The conservative Milwaukee Sentinel did the same thing in an editorial today.

Admission

Their efforts will not succeed, many party officials privately admit. "The people want to be voting in the primary where the action is, and this year the action is in the Democratic primary," one Republican said.

When the votes are counted from Tuesday's Democratic primary, Johnson supporters may claim that a large Republican cross-over to McCarthy makes the results of the primary meaningless as a test of Democratic sentiment.

Perhaps so. But it will also demonstrate that many voters--including Republicans--wouldn't like to see a Nixon. Johnson showdown in November.