After a madcap day of "yes-he-will-no-he-won't" speculation here, Sen. Edward T. Conroy today jumped into the Democratic primary for U.S. senator hours before the filing deadline, and immediately stole away some key Prince George's support from a major opponent, Sen. Victor L. Crawford (D-Montgomery).

In midafternoon, Crawford stood outside a Senate committee room pooh-poohing Conroy's candidacy, saying that he had locked up the support of "virtually all the Prince George's senators," including their former chairman, Pete A. Bozick. At practically the same time, Conroy sat in his office three flights up getting Bozick's commitment over the telephone.

"I still like Vic. I think he's great," Bozick said moments later. "But I have to go with Eddie. He's one of our guys."

Conroy's entrance into the primary, as well as the candidacies of Sen. Robert L. Douglass (D-Baltimore) and Del. Dennis C. McCoy (D-Baltimore), turned what was being called a "one-horse race" into a wild, free-for-all that left some Republicans smiling broadly. The Democrats running are vying for the chance to unseat Sen. Charles McC. Mathias Jr., the low-key liberal Republican who looms as a heavy favorite to beat any Democratic challenger.

"It's funny," said Sen. Howard A. Denis (R-Montgomery), who could hardly control an impish grin when he was told of Conroy's candidacy."I just love to see a good fight."

One Democratic legislator called all the late-comer candidates -- those who filed for office within hours of the deadlines today -- the "heart attack candidates . . . You know, they figure Mac [Mathias] is getting on in years, and he flies in planes a lot, well maybe . . ."

Mathias, completing his second six-year term in the Senate, is 57.

The entrance into the race of the two Baltimore candidates -- Douglass, who is the leader of the Senate's black caucus, and McCoy, chairman of the Baltimore City delegation -- means that no candidate is likely to have a lock on the heavy Baltimore vote, which can vault a candidate to a primary victory. A split of that vote would make it easier for a Washington suburban candidate to win the Democratic primary, several observers said.

Many believe that the Democrats' main hope for beating the seemingly invincible Mathias would be a primary that is calm, clean and never divisive. But if the first minutes of the battle today were any indication, that won't be the case.

Told that Conroy was running, Crawford, a master of the acerbic barb, said: "That's nice. Now he can be the right-to-life candidate. Now, you know, no one will vote for a single-issue man."

"I guess Ed has never gotten over his Senate presidency loss," Crawford added as a finale, referring to a stinging hair's-breadth defeat that Conroy suffered last year in the General Assembly.

Conroy's reply to Crawford's one-issue-candidate condemnation was "No way. My statement speaks for itself." And indeed, Conroy's press release, handed out as he filed for office, spoke of "exploding inflation," "stagnant economy" and "renewed dedication to social justice."

The 47-year-old Crawford is a liberal, sometime maverick veteran senator who practices law in Silver Spring and has gone off in search of higher political office unsuccessfully on several occasions.

The 51-year-old Conroy is a moderate Bowie lawyer who has championed the antiabortion cause, the D.C. voting rights amendment and various veterans causes in his 18 years in the General Assembly.

Baltimore Sen. Rosalie Abrams, who is also the state Democratic party chairman, was so taken aback by Conroy's sudden entrance into the race that her mouth flew open and her eyes began to blink wildly as she asked, "Oh dear, is he really running?" Seconds later, regaining her composure, she said, "Well, they're all credible candidates, good Democrats. It will be a good race."

While the legislators focused on their colleagues who had entered the race, there were several other candidates who filed for the Democratic primary; including John A. Kennedy, James A. Young, Mello Cottons, Richard J. Taranto, Kurt Summers, Frank J. Broschart, David E. Shaw and R. Spencer Oliver. On the GOP side, those challenging Mathias include John M. Brennan, Gerald G. Warren, Jack Fortune Holden and Roscoe G. Bartlett Jr. and V. Dallas Merrell. j

In the Eighth District Congressional race in Montgomery County, former Rep. Newton Steers will be slugging it out with three other Republicans for a chance to challenge Rep. Michael D. Barnes (D-Md.) for the seat Barnes took from Steers two years ago. Two state legislators, Del. Robin Ficker and Constance A. Morella, have filed in that primary, along with Phillip Buford.

In the Fifth Congressional District in Prince George's County, Michael I. Sprague is running in the Democratic primary against Rep. Gladys Noon Spellman, whose overwhelming reelection victories in recent years have marked her as one of the most formidable Democratic politicians in the Maryland suburbs. In the Republican primary, the candidates include William A. Albaugh, Kevin R. Igoe, George W. Benns and William Guthrie.