Loyal party warhorse Edward T. Conroy of Prince George's County amassed an overwhelming margin on his home turf last night to win the Democratic U.S. Senate nomination in Maryland and the right to face popular Republican incumbent Sen. Charles McC. Mathias.

Conroy, a Maryland state senator for the last decade, pulled together a following of Catholics, antiabortionists and veterans to beat his closest Democratic competitor by more than 20,000 votes -- a margin Conroy won in Prince George's alone.

The general election race will now pit Conroy, 51, a staunch conservative on such issues as abortion funding and defense spending, against the liberal Mathias, who easily won renomination yesterday over four lesser-known Republican opponents.

"Becoming a U.S. Senator . . . is the pinnacle of success for anybody who labors in the political fields," the jubilant Conroy said at victory celebration in Baltimore. Pressed on how he would challenge the seemingly invincible Mathias in the upcoming race, Conroy smiled broadly and said, "I want to save something for the campaign."

As the early returns came in last night, it appeared to be a head-to-head battle between Conroy and state Sen. Victor L. Crawford of Silver Spring. But then came the returns from Prince George's, where Conroy clobbered Crawford by about 23,000 votes, according to final, unofficial returns.

Conroy's overwhelming victory in his home county placed him far in front of his rivals in the crowded 12-man Democratic field, where all of the candidates were considered dark horses.

Major Democratic figures in the state such as former acting governor Blair Lee III or Baltimore City Council President Walter S. Orlinsky avoided the race against Mathias, a Republican whose popularity pierces deep into the ranks of the state's predominantly Democratic constituency.

Still, three major candidates besides Conroy and Crawford had sought to challenge Mathias -- State Sen. Robert L. Douglass and Del. Dennis McCoy, both of Baltimore, and R. Spencer Oliver of Bethesda, who had never held public office but has served the federal government in various capacities and once was president of the Young Democrats.

Conroy has gone off in search of higher office before from his position in the state legislature -- first for a congressional seat in 1972 and most recently for the state Senate presidency. The congressional quest ended with Conroy being clobbered by incumbent Lawrence J. Hogan, and his try for the prestigious Senate leadership post ended in a stinging hair's-breadth defeat.

Despite his conservative stance on an issue like Medicaid funding for abortions, Conroy was able to pull some labor support by pushing causes such as ratification of the D.C. voting rights amendment. Before jumping into the Senate race and maintaining silence on his presidential preferences, Conroy had clearly been supporting the liberal Edward M. Kennedy.

"On the day-to-day issues, Conroy votes liberal," said Del. Gerard Devlin (D-Prince George's), who represents Conroy's own Bowie area. "But when it comes to things like abortion and the death penalty, he is conservative."

During his campaign, Conroy, a Korean War veteran who lost his left arm during the fighting at Heartbreak Ridge, has pushed a strong stand on defense. "We must increase, in real dollars, the money we spend on our national defense by at least 5 percent per year throughout the decade," Conroy asserted in his issue papers on the subject.

At every opportunity, particularly in the predominantly Jewish areas of northwest Baltimore and its nearby suburbs, Conroy has made clear his "unwavering support for our only consistent ally in the Middle East" -- Israel.

His strategy in a campaign where contributors were scarce and he spent more than $20,000 of his own funds was to travel the state, seeking and often winning the endorsement of the area's elected officials, including the leaders of the old fashioned political clubs in Baltimore.

Using a time-honored Maryland ritual, Conroy's campaign gave more than $1,000 to one of those clubs, whose leaders endorsed Conroy's candidacy. The image-conscious Conroy then sought a special opinion from the Federal Elections Commissions, which ruled this week that compensation for legitimate campaign activities was legal.

But one of Conroy's opponents, R. Spencer Oliver, accused the state senator of trying to "get around Maryland's laws against" walk-around money -- the traditional term for money doled out to party workers for election day work.

Still, the resolute Conroy insisted his campaign was "meticulously clean."