Timothy M. Kennedy, a Democratic New York State senator, easily won a special House election on Tuesday to replace a retiring congressman in western New York, according to The Associated Press.

The victory was hardly a surprise. Democrats have controlled the Buffalo-area district for decades. And Mr. Kennedy outspent his Republican opponent, Gary Dickson, by an eye-popping 47 to 1.

But his victory will have an immediate impact on the House at a time when Speaker Mike Johnson of Louisiana is laboring to hold onto a narrow Republican majority and fend off a rebellion on his right flank.

Once Mr. Kennedy is seated, Mr. Johnson’s margin will effectively shrink to just a single, tenuous vote on partisan issues. A handful of special elections in Wisconsin, Ohio, Colorado and California are expected to offer Republicans reinforcements, but not until this summer.

In the meantime, Mr. Kennedy, 47, is expected to provide a reliably liberal vote. He campaigned on a familiar Democratic platform, promising to fight for federal infrastructure dollars for a region that has struggled economically, for federal abortion rights and against former President Donald J. Trump, the presumptive Republican nominee to face President Biden this fall.

Mr. Dickson, a former F.B.I. agent and local town supervisor, ran a relatively moderate campaign for a Republican in the Trump era. He had endorsed the former president, but called the Jan. 6 Capitol riot “a travesty.” He supported Ukraine’s war against Russia and federal investment in transportation projects, spending priorities that more conservative Republicans forcefully oppose.

But it was not enough to win over a district that counts more than twice as many Democrats as Republicans. With 62 percent of the votes counted, Mr. Kennedy was beating Mr. Dickson by 34 percentage points, 67 to 33.

The seat was vacated in February by the retirement of Brian Higgins, a moderate Democrat who had represented the Buffalo area for 19 years.

Mr. Higgins, who left the job early to lead Shea’s Performing Arts Center in Buffalo, was part of a wave of seasoned lawmakers from both parties heading toward the exits this year. Like many others, Mr. Higgins, 64, cited an increasingly toxic and unproductive environment on Capitol Hill.

Mr. Kennedy is a former occupational therapist who has served in the New York State Senate since 2011. In Albany, he led an important legislative committee on transportation and supported a tough package of gun safety measures after a racist shooter killed 10 people at a Buffalo supermarket in 2022. He also earned a reputation as a prolific fund-raiser.

He was selected directly by party leaders as the Democratic nominee to serve the remainder of Mr. Higgins’s term. Mr. Kennedy will likely remain in campaign mode this year, with a Democratic primary in June and November’s general election still ahead.

The district sweeps north from Buffalo, including the city, many of its suburbs and Niagara Falls.