Virginia state Sen. Omer L. Hirst (D-Fairfax) has a tough decision ahead.

Hirst now heads two highly influential Senate committees. But because a senator can hold only one chairmanship, he must decide soon which post he will retain.

For Northern Virginians, Hirst's decision will be of prime importance, since his choice will greatly affect some major issues of the area.

Hirst is chairman of the Senate Privileges and Elections Committee, which handles, among other things, voting laws, elections and nominations and appointments to any office or position in the state. The committee is scheduled to reapportion state Senate districts in 1981.

Hirst also has been acting chairman of the Senate Transportation Committee as a result of the illness and subsequent death of Sen. Paul W. Manns (D-Bowling Green). The Transportation Committee oversees the formula for distributing funds for transportation needs in rural and urban areas of the state among other things.

"It's a tough choice for Senator Hirst," said Sen. Joseph V. Gartlan Jr. (D-Fairfax).

Both committees are considered very important to Northern Virginia, and local legislators are divided on which committee Hirst should choose to remain as chairman.

"Of course, P&E (Privileges and Elections) is considered more prestigious (than the transportation committee)," said Sen. Edward M. Holland, a member of the transportation committee.

But Senate Majority Leader Adelard L. Brault said the transportation committee "is a very important committee for Northern Virginia."

Brault added, however, that the Privileges and Elections Committee also is important to the area, especially because of its upcoming reapportionment duties. The last reapportionment of Senate districts was in 1971, and Northern Virginia gained two additional Senate seats, increasing its Senate representative from three to five.

Hirst said there are some Northern Virginians who would like to see him keep the transportation chairmanship. "Some people think I should, but I haven't decided," he said.

Hirst has been chairman of the Privileges and Elections Committee since 1972. He was asked to serve as neting chairman of the Transportation Committee during the last legislative session.

If Hirst decides to give up the transportation chairmanship, which many think he will do, Sen. L. Douglas Wilder (D-Richmond) would become chairman.

However, Wilder would have to give up his chairmanship of the Senate Rehabilitation and Social Services Committee, which he said last week he is willing to do.

Sen. Stanley C. Walker (D-Norfolk) would then be in line to become chairman of the Rehabilitation and Social Services Committee.

Wilder says if he becomes chairman of the transportation committee, "the people of Northern Virginia need have no fear." He said he is "very sympathetic" to Northern Virginia's transportation needs.