The big blue Mercury gas-guzzler wheeled into its assigned parking space in the Cannon Office Building garage at 9:45 a.m. Thursday. The driver, Rep. Robert E. Bauman (R-Md.) hopped out buttoning his vest, and barely breaking stride, puffed off on the five-minute hike to the floor of the U.S. House of Representatives.

Although he has the longest regular commute in Congress, making the 154-mile round trip from Easton on the Eastern Shore every day, Bauman was in his accustomed sport near the minority table when the Rev. Edward G. Latch intoned the prayer opening the session at 10 a.m.

A House doorkeeper observed that "virtually every day" Bauman is the first member to enter the cloakroom.

Bauma explains his mission in the words of John Stuart Mill, the 19th century British philospher, who wrote that "the proper office of a representative assembly is to watch and control the government."

Or as Bauman puts it, "anytime the House is in session, America is in danger."

At age 40, the third-term member from the First Congressional District is Maryland's youngest, most conservative and probably most widely-quoted member of the House, a self-annointed parliamentary watchdog.

"He's the most valuable member of Congress," said Rep. Philip Crane (R-Ill.) "He sees that they (Democrats) play by the rules of fair play."

Speaker Thomas P. (Tip) O'Neill is less enthusiastic, saying only, "Every legislative body has a Bauman. He's tough and he certainly irks the leadership when we're trying to get things done."

The retirement in 1974 of Iowa's indomitable "no" man, H. R. Gross, catapulted Bauman from the obscurity of a freshman in the minority party to a position of informal leadership on the House floor.

"It was the consensus among conservatives that someone had to watch the store," Bauman said.

"With H. R. retired," recalled Rep. John H. Rousselot of California, "a few of us got together and asked, 'who's willing?' Bauman was." Rousselot and Reps. John M. Ashbrook of Ohio, Steven D. Symms of Ohio and Chalmers Wylie of Ohio took Bauman up on the offer and that was it. "It's not an elective office, you know," Rousselot said.

House Minority Leader John J. Rhodes of Arizona said Bauman quickly proved to be "a successor in the line of Gross and Doc Hall."

If Bauman appears at home on the House floor, it is because he has spent nearly half of his life in and around it, beginning at age 15 as a page. For the next 15 years, he worked in various capacities for the Republican side, going to Georgetown University and its lay school at night.

Walter B. Little, a 35-year House employee, has known Bauman since he was a page. "He earmarked himself to be a member since he was a kid," Little said. His attitude never changed. He was always conservative, but not against the little man. Just standing up for what he thinks is right."

And stand up he does, time and time again interrupting proceedings on technical points of which he is the House's acknowledged master.

"Mr. Speaker, reserving the right to object. I would like to propound a parliamentary inquiry to the chair." Or, "Further reserving the right to object, Mr. Speaker," and, "Mr. Speaker, I would like to move a call of the House," and then, "Mr. Speaker, on that I demand the years and nays."

Rep. Robert Dornan (R-Calif.), a fellow conservative has "this picture of Bauman at the mike, feet apart, chest up, chin out, getting off one of this best lines, such as, 'Mr. Speaker hasn't come to this, that the Speaker would sweep off his feet a minority member because he disagrees with his viewpoints?' Tip sputtered, and then went into a platitudinous speech about the rights of all . . ." laughed Dornan.

Bauman has parlayed his diligence on the floor - (he missed only six of 745 votes taken this year through Monday) - and an articulation of the conservative philosophy into a national reputation. He is widely sought out for his reaction, by correspondents who have learned they can count on him for a concise, sarcastic and often witty view of events.

One moderate Republican complains that Bauman "wants to be a darling of the conservative cause. He's on TV all the damned time."

Human Events, the weekly newspaper of the intellectual right, rates Bauman as one of 35 leading American conservatives.

Bauman clearly takes delight in expressing what liberals view as simplistic solutions to complex issues. Some examples:

The Panama Canal - "it's as much a part of America as Tablot County (where he lives). We're in control of Tablot County in perpetuity, except during hunting season, when we need a little help from the feds."

Abortion - "It's murder, I know that disturbs the emotions of some," said Bauman, who converted to Catholicism as a teen-ager, "but children are being killed by the use of federally funded abortions and that must be faced. It's the ultimatic child abuse."

Detente - "I don't say 'nuke them' by lobbing one into the john at the Kremlin, but communism is evil, always was and always will be."

Junkets - "I've never taken one, in or out of the country."

Liberal Republicans - "Democrats in drag."

Within the Maryland delegation, his closest associates are Republican Marjorie Holt of Severna Park and Democrat Goodloe E. Byron of Frederick.

"Marjorie and I confer daily," Bauman said."We try to work together on state politics because we're not on the same wave length with Steers," (Rep. Newton I. Steers, a liberal Republican of Bethesda, is the only other Republican in the Maryland House delegation.)

Since Steers and Bauman have gone to Congress, Steers said he believes "both of us have mellowed. We differ sharply phiosophically, but I like him personally. He is a most effective parliamentary gadfly. I just wish he voted differently."

Bauman gets some grudging praise from some of the Democrats in the Maryland delegation.

"Even Bauman comes on the side of the angels occasionally," said Rep. Barbara Mikulski of Baltimore. "On issues of mutual interest of Maryland, such as ocean dumping, preservation of the bay and economic development, we put ideology on the hold button."

Mikulski and Bauman serve on the Maritime and Fisheries Subcommittee, and when he is absent (which is any time the committee meets when the House also is in session), "I watch out for his interest and defend the Eastern Shore," Mikulski said.

Rep. Gladys Noon Spellman of Prince George's County said that if she has a problem with a constituent, "I can write a letter to the President or (Cabinet) secretary and know that Bob Bauman will be one of the people I can count on to back me up."

While he has gained a national following as the result of his frequent network television appearances and comments in leading conservative journals, Bauman has been tending to the home folks too.

For years, the First Congressional District of Maryland was composed of the nine counties east of the Chesapeake Bay, the Eastern Shore. But one-man, one-vote resulted in adding four counties west of the bay: Harford, near Baltimore, and Calvert, Charles and St. Mary's south of Washington.

"It's like representing three islands," said Bauman, whose secret in winning re-election has been to "keep at least two of them happy." (He lost Harford in 1974 and Southern Maryland in 1976.)

While his own polls show that his constituents are "uniformly conservative," his ideology is sometimes less apparent in positions he has taken in behalf of the First District.

He has proposed consolidating federal research on the bay (and is supported by Mikulski); rail service for the Delmarva Peninsula, and funding of the D.C. Retirement Act, because many current and former District police officers, firemen, teachers and judges live in Charles and Calvert counties.

Bauman, who acknowledges that he acquired a "smart-ass image" as an overly combative state legislator (perhaps a reflection of his admiration for his columnist friend, William F. Buckley Jr.), honed his parliamentary nit-picking skills as a state senator in Annapolis.

He always read the fine print, and was credited with discovering a printing error in one of 102 amendments offered to a 19-page land use bill that was highly desired by then Gov. Marvin Mandel. Bauman suggested the mistake might make the bill unconstitutional, forcing postponement of the vote, and a two-year delay in its passage. Bauman admitted recently that he hadn't found the error, but had benefited from a tipster who admired his careful reading of legislation.

The publicity that Bauman got enraged Bauman's fellow senator from the Shore, Democrat Frederick C. Malkus Jr., himself a symbol of Eastern Shore conservatism and independence.

They wound up running against each for Congress in a special election in 1973, after the shotgun suicide of Rep. William O. Mills. The ensuing defeat crushed Malkus, a 22-year legislator, who said last week he has "no intention" of even challenging Bauman again, and therefore had "nothing to say about him."

Mills had beaten Bauman in the GOP primary in 1971, as they sought to succeed Rogers C. B. Morton, who had been appointed Secretary of the Interior by Nixon.

In 1974, Bauman was one of 47 congressmen targeted for defeat by the AFL-CIO's political action committee (COPE). At a rally that October in Bel Air, then-Georgia Gov. Jimmy Carter denounced Bauman as someone who "exemplifies all that's wrong with the Republican Party."

Although Bauman denies having ambitions beyond his House role, Laurence E. Hulbert, executive director of the Republican Party in Maryland, believes that "Bob is very interested in running for the Senate in 1980 against Mac (Mathias)."

Bauman permits himself a sly grin and adds, "All my experience has been in the House except for the McCarthy censure hearings in 1954, when John Marshall Butler was in the Senate, (Butler named Bauman a Senate page). He was the last good senator Maryland had."