Richard Douglas, Maryland Republican, enters race to replace Democratic Sen. Barbara Mikulski

A second Maryland Republican has entered the race to fill the U.S. Senate being vacated by retiring Sen. Barbara Mikulski.

Richard J. Douglas, a lawyer who lives in Prince George’s County, announced his candidacy on Monday by noting Republican Gov. Larry Hogan’s election in Maryland, where registered Democrats outnumber registered Republicans 2-to-1.

“Larry Hogan’s 2014 victory set the stage for improvements in Maryland to American worker conditions. To move ahead, Maryland requires a new team on Capitol Hill,” Mr. Douglas said in a press release. “I am convinced that Maryland has the wherewithal to overtake Texas in job creation, unless the political machine which brought Maryland rats, riots, and the rain tax smothers urgently-needed reform.”

“In 2016, voters have the power to challenge that machine,” he said. “Maryland can send a seasoned, common-sense Republican veteran to the Senate who is eager to challenge career politicians making American workers outcasts in their own country.”

A Navy veteran, Mr. Douglas has served in the Defense Department, Justice Department and the Foreign Service, and has worked as a senior lawyer in the U.S. Senate.

In the Republican primary in June, Mr. Douglas will vie with Anthony Seda of Harford County. Chrysovalantis Kefalas of East Baltimore, a speechwriter for the National Association of Manufacturers and a former lawyer in the Republican administration of former Maryland Gov. Robert Erhlich, reportedly is exploring a GOP Senate campaign.

The winner of the Republican primary will face a formidable task in the general election in Maryland, where attention has been focused on the Senate campaigns of Democratic Reps. Chris Van Hollen, who represents Montgomery County, and Donna F. Edwards, whose represents Prince George’s County.

A noted member of the House’s leadership, Mr. Van Hollen has raised nearly three times as much money for his Senate bid as has Ms. Edwards, and both already have secured endorsements from various local politicians and organizations.

In addition, political observers are speculating that Rep. Elijah Cummings, a 10-term Democrat from Baltimore, could enter race.

Maryland’s last Republican senator was Charles Mathias, who retired in 1987 after completing his third term. He was succeeded by Ms. Mikulski, who is retiring in 2017 after serving her fifth term. Mathias died in 2010.