Armed with stacks of voter registration change cards, 12 black ministers from Prince George's County emerged smiling yesterday from the office of the board of elections in Upper Marlboro as newly registered Republicans.

They said their goal is to convince their congregations and other blacks in the county that the Republican Party offers blacks better access to political power.

"We've been taken for granted by the Democratic Party as if there was no where else to go," said the Rev. Robert Williams, president of Concerned Clergy of Prince George's County. The concerned clergy represents some 32 ministers and 20 congregations of various sizes in the Washington area.

The party switch was spearheaded by the Rev. Perry Smith, pastor of the 800-member First Baptist Church of North Brentwood, the largest of the 20 congregations.

Smith is expected to announce as a candidate for the 5th Congressional District seat held by freshman Democratic Rep. Steny Hoyer and has been talking with the Republican Congressional Campaign Committee about backing for several months. "I hope that everybody will understand that these men have made a switch based on what they think is best for their people," Smith said. "None of them is here making an endorsement of Reagan policies."

Another minister who switched, the Rev. William Smith of the Jordan Baptist Church in Capital Heights, complained that, "The Democratic Party has always chosen our leaders. This will give us a chance to choose our own." He said he could produce many new Republicans from his 350-member congregation.

Perry Smith said he hopes to register 10,000 black Republicans. The county's black population has tripled in the last decade and now represents 37 percent of the citizenry. Most of the black voters in Prince George's County are registered Democrats.