Several prominent politicians, in a campaign that appears to be gaining congressional support, have written Secretary of State Alexander M. Haig Jr. objecting to a visa for the hard-line Northern Irish Protestant leader, the Rev. Ian Paisley, who is scheduled to visit this country next month.

Paisley, who said yesterday that he will visit despite objections and plans to be in Washington about two weeks, wants to press his militantly Loyalist view of the situation in the violence-torn British province, where a Protestant member of Parliament from Ulster was murdered by Irish Republican Army terrorists last month. That attack followed a bitter, inconclusive hunger strike by Irish nationalist inmates at the Maze Prison outside Belfast during which 10 protesting prisoners died.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Sens. Edward M. Kennedy (D-Mass.) and Daniel Patrick Moynihan (D-N.Y.) wrote Haig Tuesday "to inquire whether there is any justification for Dr. Paisley to hold a valid visa" to travel to this country in view of "inflammatory appeals . . . to bigotry and religious hatred in Northern Ireland, his long-standing tactic of intimidation and oppression and his thinly veiled exhortations to sectarian violence."

Paisley, who heads the Democratic Unionist Party and is himself a member of the British Parliament, was suspended from the House of Commons for disruption last month after the assassination of the Rev. Robert Bradford, a Unionist politician. Paisley subsequently led thousands of Protestants in the province demonstrating against IRA violence.