Rep. Edward J. Markey secured the Democratic nomination in the Massachusetts Senate special election on Tuesday night and is now favored to win the seat outright.

With the Democratic establishment behind him, Markey, 66, defeated fellow Rep. Stephen F. Lynch, 58, in a primary contest that will be remembered most for the time the two were off the campaign trail in the aftermath of the Boston Marathon bombing.

In the June 25 special election, Markey will face GOP primary winner Gabriel Gomez, 47, a private equity investor and former Navy SEAL.

Markey led 57 percent to 43 percent when The Associated Press called the race with 60 percent of precincts reporting.

On the Republican side — a more unpredictable race — Gomez rode a late wave of momentum spurred by a large personal loan to his campaign. He also sought to differentiate himself as the outsider candidate.

Gomez defeated former U.S. Attorney Mike Sullivan and state Rep. Dan Winslow.

The winner on June 25 will replace interim Sen. William “Mo” Cowan, who was appointed in February to fill the vacancy caused by John Kerry’s confirmation as secretary of State.

Given the state’s strong Democratic lean, Markey starts out as the heavy favorite. Still, Gomez’s fundraising ability and recent special-election history in the state will force Democrats to take the race seriously.

In the January 2010 contest for the seat of the late Sen. Edward M. Kennedy, then-state Sen. Scott P. Brown, a Republican, was elected to the Senate in a shocking upset. Three years later, Democrat Elizabeth Warren defeated Brown when he attempted to win a full term in November 2012.

The Democratic primary candidates combined spent $5 million through April 10, the end of the pre-primary fundraising period. Markey’s campaign spent $3.2 million of that total. He led the fundraising race and had $4.7 million left to spend with three weeks to go. Lynch had just $514,000 in cash on hand at that same point in time.

The campaigns spent very little of that money in the days following the April 15 terrorist attack at the finish line of the Boston Marathon. All five candidates suspended campaigning during a week that included a visit from President Barack Obama at a nationally televised interfaith service and a manhunt that locked down all of Boston and some neighboring towns.

With one suspect dead and another in custody, Markey and Lynch returned to the airwaves and reignited their ground campaigns on April 22. They also squeezed in back-to-back debates last week that focused far more on national security than they otherwise would have.

Still, on television and in the debates, Markey continued to highlight his liberal stances on the environment, abortion rights and gun control. Lynch is the most conservative member of Massachusetts’ all-Democratic congressional delegation.

The Markey establishment support came from Kerry, the Kennedys and the Democratic Senatorial Campaign Committee. He also received more than $400,000 worth of field program assistance from the League of Conservation Voters.

It was Markey’s first competitive election since winning a 12-candidate primary in 1976. It was Lynch’s first special-election primary since Sept. 11, 2001, when he secured the Democratic nomination and won his seat three weeks later.