Virginia Gov. Charles S. Robb came as a novice, with barely five weeks of on-the-job training under his belt. His Maryland counterpart, Gov. Harry Hughes, was by comparison an old veteran, nearing the end of his four-year term.

And the three-year gap in their experience appeared to make all the difference, as the two governors played sharply contrasting roles in this week's pivotal meeting of the National Governors Association.

The two men sat through the same meetings with President Reagan and congressional leaders. And they both joined 34 other governors in soundly rejecting key portions of Reagan's New Federalism--the sweeping proposal to shift control over dozens of domestic programs from Washington to the nation's 50 statehouses.

But in almost every other respect, Robb, the newcomer, and Hughes, the incumbent seeking re-election, assumed sharply different poses during yesterday's often-tortuous talks about how governors can persuade the president to moderate his budget cuts and reshape the proposed New Federalism.

Hughes spoke as a governor who spent the fall and winter cutting tens of millions of dollars from his state budget, lopping families off welfare and revamping the state's whole day-care system. While Hughes was taking those steps to absorb the Reagan cuts--amid loud protest from advocacy groups--Robb was stumping Virginia as a candidate.

The 55-year-old Hughes, who has loudly criticized Reaganomics for the last several weeks, spoke sternly in an interview yesterday about the "insensitivity" of the Reagan cuts and about the "chaos" those cuts have brought to Maryland in the last year. The Virgnia governor, by contrast, spoke of his need to learn as he goes, to watch and wait. "I'm willing to give the president the benefit of the doubt," said the 42-year-old Robb.

When the two governors sat together on a panel with Senate Budget Committee Chairman Pete Domenici (R-New Mexico) and House Budget Committee Chairman James R. Jones (D-Okla.), Hughes pressed the two fiscal leaders for details of how much Congress is likely to cut in federal aid to the states, complaining of the uncertainty that now plagues his budget staff. Robb raised his hand at the end to ask what he called a "quick philosophical question" about how large a federal deficit the economy can absorb.

Hughes, a 25-year veteran of state government, chaired a key transportation policy committee that drafted one plank of the governors' rebuttal to Reagan's federalism. It proposed separating funding for 11 highway and mass-transit programs from a proposed $28 billion trust fund that would help states pay for their new programs. Robb, who came to the governor's office with only four years of state government experience--as a part-time lieutenant governor--took part in committees as a rank-and-file member.

"I'm one of only two new governors here," Robb said. "I think it'd be presumptuous to come in and attempt to exert undue influence at this point."

But Hughes appeared to feel well-qualified to attack the president's policies. "When you prepare a budget, when you try to deal with these very serious cuts, when you're forced to change whole programs in midstream because of new federal cuts--you get very concerned," the Maryland governor said.

Hughes' stronger anti-Reagan stance grows in part from a more liberal philosophy than Robb's. But it also reflects the strong bipartisan opposition among most governors to further budget cuts and to much of the New Federalism--opposition brought on because of the difficulty they faced in making budget cuts during the first full year of Reaganomics.

The bipartisanship was evident yesterday when only five governors voted against the National Governors Association statement opposing much of the Reagan federalism. The association's Republican president, Richard Snelling of Vermont, has been one of the most outspoken critics of the cuts.

Despite their difference in tones, both Robb and Hughes are presiding over similar problems related to the Reagan cuts, the recession and persistent high interest rates. Both states are suffering from increasing unemployment and shrinking state revenues.

At the end of the day, the two went with their fellow governors to meet Reagan in the East Room of the White House, and to hear him agree to negotiate on most of their objections. When they emerged, newcomer and veteran sounded alike for the first time all day: Assured that Reagan would at least consider their pleas, they said they were relieved.

"They main thing is that he's willing to negotiate. That gives me some hope," said Hughes.

"Anytime you don't leave with a complete impasse you're encouraged," said Robb. "It surprised a number of us."