Maryland's presidential primary campaigns gained a measure of last-week intensity yesterday as George Bush took his never-say-die campaign to Baltimore and Jimmy Carter's coordinators declared that they had decided to redouble their efforts in the state.

Bush, making his first stop in Maryland to stump for the May 13 primary, refused to relinquish a tone of determined optimism as he greeted small knots of well-wishers at the city's annual Flower Market and in the streets of Little Italy.

"I'm doing this because I believe I'm going to win," Bush declared as he munched roast pork and rye bread at Russo's Restaurant. Of the pollsters and political writers who give him little chance of overtaking Ronald Reagan after Reagan's victories in three primaries Tuesday, Bush said: "I just can't be discouraged about what they say."

Carter's resounding wins Tuesday in Indiana, Tennessee and North Carolina prompted his strategists to order a media campaign and more appearances by the president's surrogates in Maryland in the hope of delivering another painful blow to the campaign of Sen. Edward Kennedy next week.

"We've decided within the last 24 hours to substantially increase our effort," said Tim Finchem, a deputy campaign manager in Carter's national headquarters. "We thought for a long time that Maryland was a Kennedy state -- the suburbs are tough -- but given the Indiana results we're feeling a surge of momentum out there and we want to capitalize on it."

Kennedy's Maryland coordinator, Ernie Essler, said yesterday that his staff's telephone polling last week showed Carter leading Kennedy in the state, but that a substantial block of voters -- as high as 60 percent -- was still undecided.

Both campaigns planned to begin media advertising blitzes this week.The Carter campaign, which had not orginally planned to spend a substantial amount in the state, will now pay out more than $40,000 for television spots in Baltimore and rural Maryland, while Kennedy coordinators are buying around $25,000 in television in the Washington and Baltimore markets.

Bush, however, will outspend all other presidential candidates in the state. His regional campaign director, Dorann Gunderson, said yesterday that about $100,000 will be spent in Maryland, including $31,000 for 30-second advertisements that began running on three Baltimore television stations Tuesday.

In addition, a mailing was sent out last weekend to about 125,000 of the state's 419,000 registered Republicans, a mailing designed to reach Republicans who regularly vote in primary elections and who live in the regions around Washington and Baltimore.

Bush, looking slightly gaunt but nonetheless energetic, spent 4 1/2 hours campaigning in Baltimore before heading for a reception at his Silver Spring headquarters.

During a half-hour appearance on WFBR radio's Conference Call, Bush sharpened some of his criticisms of the Republican front-runner's position, at one point labeling Reagan's call for a blockade of Cuba a "dumb idea."

At another point, noting his disagreement with Reagan's proposal to dismantle the Federal Energy Department Bush said, "Maybe I resist over-simplification." He called again for Reagan to disclose his income tax returns.

But throughout the afternoon, Bush reserved his harshest criticisms for President Carter and reiterated his intention to support Reagan if the former California governor wins the GOP nomination.

Today's campaigning began a two-day swing through the urban and suburban areas of Maryland, where 30 delegates are at stake in next Tuesday's primaries.