Rep. Clarence D. Long (D-Md.) started his 11th term in Congress this year by changing his mind. He let it be known that he no longer opposed the dredging of Baltimore Harbor, a project he had single-handedly blocked for a decade, and he announced to stunned Democrats that he no longer thought it was such a bad idea to increase military aid to El Salvador.

The switch on harbor dredging was popular, since state officials want to open up the Baltimore port to larger ships. The switch on El Salvador--and Long's emergence as a seemingly key figure in Reagan administration foreign policy, making quick, dramatic flights to Central America--has stirred a batch of controversy.

"When you try to do the right thing, you get in trouble," Long, 74, protested yesterday, shortly after the House Appropriations Committee's subcommittee on foreign operations, which he chairs, had voted to give the president half of the $60 million he requested. "I'm just trying to move toward peace."

Long, who used to say that El Salvador could become another Vietnam, said he changed his mind on providing military aid because he fears a right-wing military coup, because the country now has an elected government and because he sees an improvement there in human rights.

"I think we should have stayed out, but we're there now," he added.

Long's role in the El Salvador question has won praise from most of those who support the administration position.

"He deserves credit for the time he's spent on the issue. I don't think there would have been any assistance to El Salvador were it not for him," said Rep. Jack F. Kemp (R-N.Y.), the ranking minority member of the subcommittee.

But others, including some who favor of increased military aid, say the turnaround by the former Johns Hopkins University economics professor is just the latest indication of an eccentric political style.

In the course of his recent stint in the limelight, the blunt representative from Baltimore County has been called everything from a "brilliant maverick" to "cantankerous" to an "egomaniac." Some say Long's major talent on Capitol Hill is for getting his name and photograph in the newspaper.

"He's only called one Maryland House delegation meeting so far this year--and that was to take a picture," complained one critic.

"Doc" Long, who vies with Republican Sen. Charles McC. Mathias Jr., also a 21-year Hill veteran, for the title of dean of the 10-member Maryland congressional delegation, has had a stormy relationship with his colleagues in general and committee members in particular.

In the delegation, there are stories of his coming late to meetings, elbowing his way into camera range, making self-congratulatory remarks and leaving.

On the committee, he has had several run-ins with staff and members. Several weeks ago, Long's attempt to cut off subcommittee debate in a session dealing with El Salvador brought an outburst from Rep. David R. Obey (D-Wis.), who angrily asked him, "Has the chairman lost his mind?"

He is best remembered, however, for his gaffes, especially the time three years ago when he told an NAACP gathering that blacks were "genetically superior" to whites because "Southern white blood flows in every black today."

Long is also accused of costing the Maryland money because of the delays he has engineered on such projects as the harbor dredging and the second span of the Chesapeake Bay Bridge. And there are delegation staffers who say he tried to keep other Marylanders off the Appropriations Committee, a charge Long has denied.

But whatever the criticisms, Long enjoys strong support in his suburban district, where he can be seen out jogging and where his constituent service is called "outstanding." Labor unions and Jewish voters love him.

There is talk that he may have a tough reelection, but even Republicans scoff at that. Says one GOP aide, "He's articulate, and he does his homework. It's a safe district--he's never going to lose it."