A Somerville man is expected to challenge incumbent state Sen. A. David Trahan for the seat that covers Lincoln County, as well as part of Knox and Kennebec counties.

Christopher Johnson has filed a declaration of intent with the Maine Commission on Governmental Ethics and Election Practices to seek the Democratic nomination for the Maine Senate District 20 seat. He is seeking to be a publicly financed candidate.

Johnson is vice chairman of the Lincoln County Democratic Committee and has served on town boards.

The seat is held by Trahan, a Republican from Waldoboro, who is serving his first year in the Senate after four terms in the House.

Senate District 20 encompasses all of Lincoln County, the Knox County towns of Washington and Friendship, and the Kennebec County town of Windsor.

The deadline for party candidates to file nomination papers with the Maine Secretary of State is March 15. The party primaries will be held June 8.

Candidates not enrolled in any party have until June 1 to file nomination papers. The general election is Nov. 2.