For his part, Mr. Gomez repeatedly cast Mr. Markey, who was first elected to Congress in 1976, as a relic who had little to show for his decades in Washington. He cast himself as a “new kind of Republican” who supported same-sex marriage, expanded background checks for gun purchases and a path to citizenship for illegal immigrants. But he could not convince enough Massachusetts voters that he would vote independently of the national Republican Party.

While the Markey campaign feared that Mr. Gomez would attract millions of dollars in outside money from special interests, that backing did not materialize in a serious way.

Mr. Markey himself did not exactly set the electorate on fire. He was the consensus choice of the Democratic establishment, which wanted to focus on one candidate early in hopes of avoiding a repeat of 2010, when a Republican, Scott P. Brown, won the special Senate election to fill the seat of Edward M. Kennedy, who died in 2009.

Mr. Markey ran a cautious campaign that relied heavily on big Democratic stars, including President Obama, Michelle Obama, former President Bill Clinton, Vice President Joseph R. Biden Jr. and Senator Elizabeth Warren.

“Democrats are getting the outcome they were supposed to get, but they had to put a lot of firepower into it,” said Jennifer Duffy, a senior editor at The Cook Political Report. She characterized the Democratic juggernaut, including an extensive field operation that made contact with three million voters over the last five days, as backup insurance that the loss of 2010 would not be repeated. On Tuesday alone, the party organized 7,200 volunteers to get out the vote.

“This victory has less to do with Ed Markey and more with the fact that he’s the Democrat,” she said. The Democratic Senatorial Campaign Committee took considerable credit for the victory Tuesday night.