After lobbying by Gov. Marvin Mandel and a chorus of appeals to "responsibility," the Maryland Senate today reversed its surprised decision of Monday night and easily approved the proposed budget for the 1977-78 fiscal year.

Before voting for the budget, however, several senators on whose votes today's decision turned said they are adamant about not supporting the measure that would fully fund the budget, the one-percentage-point increase in Maryland's current 4 per cent sales tax proposed by Mandel.

Although today's 29-to-18 vote in favor of the budget was preceded by the usual series of impassioned speeches, most of the seven senators whose votes changed in the last 48 hours indicated that their votes to approve the measure that funds all government spending were inevitable.

"It wasthe responsible thing to do," said Sen. James Clark Jr. (D-Howard County), who voted against the budget on Monday. "I'm still not happy with the budget. We should have made more cuts than we did.

"But you can't hold up the budget. You have to go ahead. I think everybody that voted against it knew it would have to be reconsidered," Clark.

"I felt that ultimately, if this did stick, the state would be stuck without a budget," said Sen. Howard Denns (R-Montgomery), another who changed his mind and supported the budget today. "I think they got the message."

The message many senators wanted to send Mandel was twofold - first, that they do not want to vote for a sales tax increase in a year when they are inundated with constituent complaints about taxes, and second, that they are displeased at their inability to significantly affect the budget process.

"I just can't believe" there was no room for more than $4.5 million in mostly insubstantial cuts the Senate made in three days of fighting over the budget, said Sen. Peter Bozick (D-Prince George's).

"I'm so frustrated - sometimes I feel like I'd like to walk up to somebody and hit him," said Bozick.

It is not clear, though, that the Senate wanted to send its message to Mandel in terms as strong as the disapproval of the budget.

Monday night's vote against the $3.9 billion budget apparently caught everyone from the Senate leadership to the legislators who organized the opposition by surprise. A strong vote of protest was expected, said Senate President Steny H. Hoyer (D-Prince George's); rejection of the budget was not.

Hoyer said he did not even check to make sure he had enough votes to pass the budget before it came to the floor Monday. "There was no reason to expect the budget would be defeated," he said. "I've never seen a budget defeated. Every member of the Senate thought the budget was going to pass Monday night."

Majority Leader Roy N. Staten (D-Baltimore County) said two senators who voted against the budget Monday came to him immediately after the Baltimore County) said twosenators vote and said they would have supported the budget if they had suspected it might fail.

Sen. C. Lawrence Wiser (D-Montgomery) voted against the budget Monday, but said he "made my conclusion to go for the budget almost immediately that night."

Wiser said he was afraid Mandel might cut aid to the counties if forced to trim his budget proposal by unfavorable Senate action. "The prospect of that scares me enough tomake the difference in my mind," he said.

Not all of the vote changes were spontaneously induced, according to thh angry speeches of several senators who decided not to change their votes today.

"In two days, the threats are made arms are twisted, and the votes are turned," said Sen. John C. Coolahan (D-Baltimore County), a leader of the move to defeat the budget.

"It's unfortunate that after 20 years, we have a monarchy (ruled by Mandel) here, and some of the legislators are becoming vassals."

Those who changed their votes - including Bozi K and Sens. Homer White (D-Eastern Shore), Clarence M. Mitchell 111 (D-Baltimore) and Edward T. Hall (R-Calvert) game various reasons for doing so. None said he gave in to pressure.

The man who took the defeat of the budget most seriously was Hoyer. It provided an opportunity for his leading opponents for the gubernatorial nomination to take pot shots at his leadership, and for Republicans to needle him. He responded: "This is not my budget. The premise that every major bill that comes down automatically becomes my bill is incorrect."

The budget will now go to the House of Delegates, which is expected to make substantial cuts beyond the cuts made by the Senate.