Maryland Gov. Harry Hughes today signed into law 100 of the bills enacted by the 1985 General Assembly and basked in the accomplishments of a session that will certainly add sparkle to his political resume should he decide to run for the U.S. Senate in 1986.

After signing, among other pieces of legislation, a measure giving the Maryland thoroughbred racing industry a $12 million shot in the haunch and another that places an immediate moratorium on new hospital construction, Hughes said that the 90-day meeting that ended Monday night was "one of the most successful sessions for the people of Maryland in a long time."

Among the measures passed by the General Assembly were most of the governor's "youth initiative" program, other bills to help contain the cost of health care, a ban on the sale of phosphate detergents, bills allowing Maryland to enter into a regional banking compact and the New York banking giant Citicorp to expand in the state, and increases in welfare benefits.

Hughes, who cannot succeed himself when his second term expires in January 1987, had promised repeatedly to announce his political intentions after the conclusion of this legislative session. Today, Hughes said he would make good on that pledge -- later.

"I haven't made any final plans," said the 58-year-old Hughes, looking trim and ready for a campaign after shedding 17 pounds. But Hughes added that "it would be less than honest" to suggest he has not considered a run for the U.S. Senate seat held by Republican Charles McC. Mathias Jr.

Observing the usual proprieties on the day after a legislative session ends, Hughes shared much of the credit for what was accomplished with the presiding officers of the Senate and House of Delegates. "The important thing in a legislative session is what you accomplish, not necessarily who gets the credit."

But that spirit of collegiality masked some strains that developed during the last 90 days, particularly between Hughes and House Speaker Benjamin L. Cardin, who is positioning himself for a gubernatorial bid in 1986, and between Cardin and the increasingly powerful Senate president, Melvin A. Steinberg.

Cardin and Hughes jockeyed for position and credit -- usually politely -- several times during the assembly session, but both made attempts to smooth over differences.

Hughes, in a gesture that said much about his more activist posture this session, paid at least two visits to Cardin's office in the final two weeks to conciliate differences and shape strategy as the legislature contended with a host of major issues in the final days.

The governor also shared the Senate rostrum on the final night with Steinberg, whose lack of immediate political ambition has contributed to Hughes' obvious fondness for the Senate president.

The racing bill signed into law by Hughes today provides a $12 million tax break to a thoroughbred industry that is fighting for survival against tracks in nearby states, particularly New Jersey. The legislation earmarks much of the savings for physical improvements and higher purses.

Another important piece of the Hughes administration's legislative package also was signed and will take effect immediately. One of seven bills crafted to reduce the rising cost of health care, the ban on hospital construction that contains a number of specific exemptions will be in force until October.