JEFFERSON CITY -- The campaign to sell state lawmakers on a $500 million highway construction bond issue has lost a passenger: Sen. Emory Melton, a 23-year veteran legislator whose fiscal conservatism has often led the General Assembly on a right turn.

The Southwest Missouri Republican has told friends he would have "difficulty" in exchanging $760 million -- the cost of the bond issue plus additional costs for interest and insurance -- for $500 million in road improvements. As the ranking GOP member of the Senate, Melton has served in the upper chamber since 1972.

Melton is expected to wield wide influence on the bond issue proposal because of his seniority and membership on the Senate Appropriations Committee and the State Budget Control panel.

Recently Melton said the plan to issue $500 million in state bonds to complete funding of a $2.6 billion road improvement plan is a "matter of concern" since "we are trading off $260 million worth of highway construction for a quick fix in the 15-year program." He added: "All $760 million will be paid out of highway funds, which will be taken from maintenance and construction. The only place it could come from would be a tax increase."

Melton praised efforts of the Highway and Transportation Department's chief engineer, Joe Mickes, in trying to salvage the original building schedule, which the agency has said was too optimistic and underfunded. The senator described Mickes as "one of the most respected department directors in state government."

Mickes met recently with 11 Southwest Missouri legislators in an effort to coax their support for the bond issue proposal. The chief engineer asked the lawmakers to "give me a chance," and most attending the session seemed willing to do so. One legislator, Rep. Delbert Scott, a Lowry City Republican, responded, "As long as you tell us the truth, you'll be all right with us."

Another area representative, Ken Legan, a Republican from Halfway, said if the bonding issue is approved, it will be a tribute to the chief engineer, who has worked hard in recent months to restore confidence in the transportation agency.

One of the most enthusiastic supporters of the plan is the senator who heads the Transportation Committee, Danny Staples, an Eminence Democrat who owns and operates a resort on the Jack's Fork River. Staples has been contacting fellow senators, seeking their support for the funding plan that would bring improvements to routes leading to his resort.

Melton says he has misgivings despite the fact the plan would provide highway construction totaling $181 million in his senatorial district. But he says: "These projects are not solely dependent on the $500 million bond money. Work will continue out of the $2.1 billion available. Some of the work may be delayed."

Several legislators in northwest Missouri, where there are fewer planned new projects than in most other areas of the state, have taken the same cautious approach as Melton.

The construction program that began last April and will extend through 1988 would be financed with $850 million in state highway user taxes, $1.25 billion in federal funding and the proposed $500 million bond issue.