Hunter B. Andrews, a Virginia state senator known for his wit and skill in debate, yesterday became the third Democrat to declare for the 1978 nomination to the U.S. Senate.

Andrews, a 56-year-old Hampton native, said, "I beleive my 20 years of public service locally and in the state Senate equip me to represent all Virginians in the U.S. Senate."

Democrats Frederick A. Babson of Virginia Beach, a former Fairfax County Board chairman, and former Fairfax Supervisor Rufus Phillips have announced that they are running for the party nomination, while former Attorney General Andrew P. Miller, U.S. Rep. Herbert E. Harris, Del. Carrington Williams of Fairfax and state Sens. Clive L. Cuval II of McLean and James T. Edmunds of Lunenburg are other possible contenders.

Republicans who have announced for the seat now held by Republican Sen. William L. Scott are former state GOP chairman Richard D. Obenshain and state Sen. Nathan Miller of Rockingham. Former Gov. Linwood Holton and former Navy Secretary John Warner also are expected to fun.

Andrews, a graduate of William and Mary and the University of Virginia Law School, is considered a conservative, but often takes stands on individual issues that make him difficult to catalogue.

As chairman of the Senate Education Committee, he is one of the state's most influential persons on matters affecting Virginia's schools. He has voted with libertals to block attempts to interject political views into teaching a state universities.

Andrews, a World War II Navy veteran and a director of the Virginia National Bank of Hampton, voted for the Equal Rights Amendment, but sided with conservatives when moderates took over the Democratic leadership in the state Senate two years ago.

First elected to the Senate in 1964, he generally is considered one of the finest debaters in that body and is a master of rules and procedure.

Andrews might gain support from his home area and from conservative Democrats in the General Assembly. He did not support Democrat Henry Howell for governor and possibly will encounter opposition from the partys' liberal wing.

In announcing his candidacy in Hampton, Andrew said ". . . I believe we must have a candidate who can attract Democrats and Virginians who believe in sound government.

"There is a Virginia way to help America - progress through honesty, work and fiscal integrity. I hope you will join me so together we can carry the Virginia way to Washington," Andrews said.