Henry E. Howell, the combative former leader of Virginia's liberal Democrats, marched on the state Capitol today to warn Gov. Charles S. Robb against straying from a major campaign commitment to the state's black citizens.

What began as a protest turned into a lovefest, however, when the colorful former lieutenant governor and a band of loyalists he called "Howell's army" emerged from a 30-minute meeting with the governor and proclaimed that Robb had gotten their message.

Despite earlier signs to the contrary, Howell said Robb had given an "enthusiastic reendorsement" of his campaign pledge to back postcard voter registration to increase black voting in the state. "He's done everything he can," Howell said. "The purpose of our trip to Richmond has been uniquely successful."

That view was later challenged by a number of key lawmakers who said proposals for a constitutional amendment to allow postcard registration stand little chance this year in the conservative General Assembly. But, some legislators added, Howell's trip may have had other purposes.

Shunned by his party during the 1981 governor's race and disaffected from the mainstream of state Democrats, Howell's championing of the postcard issue gave him an opportunity to thrust himself back into a statehouse scene he departed after losing to Gov. John N. Dalton as the Democratic Party's standard-bearer in the 1977 gubernatorial campaign.

Just hours before his meeting with the governor, the silver-haired, 61-year-old Howell had stood outside the historic St. John's Church about a mile from the Capitol and told his group of about 20 shivering loyalists that he was disappointed that Robb had not proposed postcard registration during his "State of the Commonwealth" address to the legislature.

"Our governor made a commitment on the campaign trail," Howell proclaimed. "We are here to make sure that commitment becomes a reality."

Invoking Patrick Henry, George Mason and his own longtime fight to abolish the state poll tax, Howell said postcard registration was needed "to pay penance" for the decades during which Virginia systematically excluded blacks from the voting rolls.

"If we get a deaf ear from those we petition today, then I'm afraid they'll get a deaf ear when they want our vote to elect Democrats to the United States Senate and the House this year," said Howell, a Norfolk lawyer who ran three times for governor.

During his speech, Robb had proposed an alternative plan to increase black voting in the state by allowing registration at offices of the State Division of Motor Vehicles. That is a "poor solution" that would do nothing to help the elderly and infirm, Howell said today. "It's not worthy of consideration as far as I'm concerned," he added. "Who wants to queue up behind another line that's been queueing up in an office where they're understaffed anyway?"

Howell's army then began their march to the Capitol, carrying colored oversize postcards drawn by Howell's daughter saying "Dear Chuck, Our Guv, If you need our Luv, give us postcard registration. Thanks, John Q. Public." Among the marchers was C. R. Smalley, a wizened Democratic Party organizer from Chesapeake who loudly reminded listeners that he had worked who supported Baliles in the November elections. campaigns during the past 20 years. "If he had only beaten that mug-wump [Gov. Mills E.] Godwin, we'd of had it made today," he said.

Before Howell's protest movement could get off the ground, Robb moved quickly to defuse the issue. He told statehouse reporters at an impromptu press conference on President Reagan's latest budget proposals that "I still support" postcard registration, but that due to General Assembly opposition he believed his motor vehicle plan "is more likely to be achievable."

Later in the afternoon, Howell brought his bank of liberal loyalists along to a meeting in the governor's office. Robb, who long had chilly relations with Howell, had originally agreed to the meeting several weeks ago, thinking it would be a private chat, said press secretary George Stoddart. "I guess this is Henry's way of making a point," he said.

Postcard registration proposals have long been pushed by civil rights groups in Virginia, which charged that the state is 48th among the 50 states in in its percentage of eligible citizens who are registered. They say this is because of state laws that make it difficult to register.

Postcard registration is resisted by such legislators as Del. Claude W. Anderson (D-Buckingham) the chairman of the committee that will consider the bill Howell is backing. "I'm not going to vote for anything that would open the doolr to any fraud," Anderson said today."A lot of people could get a postcard and put whatever they wanted on it. You could put people's names on it that don't even exist."