New Jersey Gov. Thomas H. Kean (R) appears headed for an landslide reelection victory today, and Republicans hope his coattails will be long enough to give them control of the state Assembly.

Kean has had a wide lead over Democratic challenger Peter Shapiro in all preelection polls, with unusually strong support from blacks, unionists, city dwellers and others often alienated by the GOP's national policies.

If Republicans take over the New Jersey Assembly, it would move the GOP one step closer to its goal of controlling enough state legislatures to influence congressional redistricting after the 1990 census.

But Democrats also expect to crow after today's state and municipal contests around the country, where incumbency, economic resurgence and moderate politics generally seemed to take precedence over ideology, party or the political shadow of President Reagan.

Polls indicate the Democrats appear poised to sweep the top state offices in Virginia, which joins New Jersey in the only other governor's race held this year. Democrat Gerald L. Baliles is favored to succeed Gov. Charles S. Robb (D), and Virginia voters appear on the brink of electing the state's first black lieutenant governor and woman attorney general.

Those would mark the only significant gains this fall for women and racial minorities in their pursuit of elective office. Some observers attribute this situation to the small number of elections this year year and to the "saturation factor" -- most jurisdictions where blacks are a significant portion of the voting population already have elected black officials to represent them.

Several minority politicians already have been reelected this year -- or are expected to win additional terms in voting today. They include mayors Coleman A. Young in Detroit, Tom Bradley in Los Angeles, Andrew J. Young in Atlanta, Harvey B. Gantt in Charlotte, Henry G. Cisneros in San Antonio.

The liveliest mayoral election is in Houston, where incumbent Mayor Kathy Whitmire is being challenged by former mayor Louie Welch. The race has drawn national attention as an indicator of how acquired immune deficiency syndrome (AIDS) may cut as a political issue. Whitmire was elected in 1981 with strong support from Houston's homosexual community, and Welch has pitched his campaign to play on fears of a possible AIDS epidemic and anti-homosexual attitudes among some voters.

If Whitmire is reelected, six of the nation's 10 largest cities will be run by black, Hispanic or women chief executives. All are Democrats. New York Mayor Edward I. Koch is considered a shoo-in for a third term. The Justice Department announced yesterday it would send 107 federal observers to New York to make certain the rights of Latino voters are protected. It marks the first time federal observers have been sent to New York City.

In Miami, Puerto Rican-born Mayor Maurice A. Ferre faces his toughest challenge, primarily from two Cubans and one black. A Nov. 12 runoff is expected.

Incumbents on all levels and from both parties cruised through campaigns on their records, generally espousing fiscal conservatism but social compassion, and reaping the benefits of economic recovery.

This year, despite the rhetoric of national party operatives, Republicans running for major office tried primarily to run on their own, and Democratic opponents shied away from political attacks on the president.

Reagan essentially restricted his campaign stumping to one fund-raiser each for Kean and Virginia GOP gubernatorial nominee Wyatt B. Durrette.

More than two-thirds of the governorships, all 435 House seats and one-third of the seats in the U.S. Senate are up next year, and both parties were eager yesterday to place today's anticipated results in the context of those contests and the 1988 battle for the presidency.

Democrats focused their comments on Virginia, claiming that their anticipated top-to-bottom victory in a race with no incumbent proved that realignment -- the shift in voter preference from Democrat to Republican -- was a folly, even in southern states where Republicans have said such a shift is most evident.

"The big message that comes out of Virginia," said Democratic National Committee spokesman Terry Michael, "is that when Democrats campaign on mainstream issues connecting with the average voter, we win.

William Greener, deputy chief of staff for political operations of the Republican National Committee, said the results of today's contests would not make "an outrageous case one way or the other for realignment, dealignment or anything else."