In Ohio's 4th Congressional District, Mike Oxley is under fire from his main opponent in Tuesday's primary election because a year ago he supported the man Ronald Reagan eventually chose as his vice presidential running mate.

Oxley, a 37-year-old state representative, supported George Bush for the 1980 Republican presidential nomination, a fact he is constantly reminded of by Robert J. Huffman, 49, a former Miami County prosecuting attorney and longtime Reagan supporter.

Huffman and Oxley are the leading contenders in a field of six vying for the GOP nomination to replace the late Tennyson Guyer in one of the most Republican congressional districts in the country. Guyer, a Republican, died of a heart attack April 12.

There are, aides to both candidates say, no substantive issues because the two men agree on virtually everything, from abortion and gun control (they are against both) to the Reagan economic program and the need for a strong defense (they are for both). The campaign, in the words of one observer, has come down to a contest between Oxley's attempt "to run with Tenny Guyer's ghost" and Huffman's effort to portray himself as "the Reagan candidate."

Oxley is running with the support of Guyer's widow, May, and recently was endorsed by former U.S. attorney general and former U.S. senator William Saxbe, a popular figure in this area. Oxley has stressed his experience in the state legislature, arguing that this makes him better qualified to take on the duties of a congressman.

Huffman has countered by attempting to turn the race into a test of Reagan loyalty, in which he portrays Oxley as a latecomer to the conservative cause.

In telephone interviews last week, Ohio political figures generally gave the edge to Oxley. He is from Findlay, in the northern section of the district, while Huffman's home is in the southern portion of the district. The battleground area is around Lima and Allen County, which GOP county chairman Bob Holmes said he expects Oxley to carry.

The winner of the Republican primary is expected to face Democratic state Rep. Dale Locker in the June 25 general election. Locker, who last won reelection in the heavily Republican area with 72 percent of the vote, faces only token opposition in the Democratic primary.

The Ohio primary has not only been devoid of national issues, but it also has been expensive for a congressional contest in that area, with Oxley and Huffman each expected to spend about $100,000. In that respect, it has much in common with the primary campaigns for governor of New Jersey, which also will be decided by the voters on Tuesday.

In New Jersey, the spending per candidate has to be counted in the millions, not thousands. Under the state's new public financing law, more than $1 million in public funds is available to candidates in both the Democratic and Republican primaries, most of whom have not been shy about accepting it.

The result has been a confusing array of 13 Democratic and eight Republican candidates seeking to succeed Democrat Brendan T. Byrne, and much uncertainty about the outcome of both primaries.

But going into the final weekend, the leading Democratic contenders were considered to be two congressmen, James J. Florio and Robert A. Roe, state Attorney General John Degnan, state Senate President Joseph Merlino and Jersey City Mayor Thomas F. X. Smith.

The top Republicans, state party officials say, are Paterson Mayor Lawrence F. (Pat) Kramer, former state Assembly speaker Thomas H. Kean and Joseph A. Sullivan, a wealthy businessman who declined public financing and had spent $1.8 million with more than a week to go before the primary.

There have been no issues to speak of in the New Jersey contests. Merlino is thought to have made some strong gains by stressing gun control in the Democratic contest, but he started far behind.Roe, who also refused public financing, has made the public funding a main issue and may profit from what is believed to be widespread public disgust with the new system.

Dave Earling, executive director of the state Republican Party, said organization, rather than news media advertising, may be the key in both primaries, in which case Roe, in the Democratic primary, and Kramer, in the GOP contest, should benefit.

"When you've got as many candidates as we do, people get tired of getting hit over the head with media and you turn them off," Earling said. "When your TV program gets interrupted five times in an hour by some guy pushing himself for governor, you can have overkill."

While the outcome of the primaries remained uncertain, Earling said there was little disagreement about the merits of the public financing law.

"The average voter is terribly confused," he said. "The people who came up with the public financing law ought to be hung out to dry."