Senate Minority Leader Harry M. Reid (D-Nev.), pictured Sens. Patty Murray (Wash.) and Charles E. Schumer (N.Y.), responds to a question from the news media during a briefing at the Capitol. (Shawn Thew/European Pressphoto Agency)

The formidable political machine Harry M. Reid built in Nevada will face its most difficult task to date when it tries to hold on to the Senate seat being vacated by the retiring minority leader next year.

After getting crushed in the 2014 midterm elections, Senate Democrats have a chance to win back the majority a year from now. If successful, they would set the chamber’s agenda and, more important in the short run, control the nomination process for the next president’s Cabinet. That would require a four-seat gain if the Democratic candidate wins the presidency or a five-seat gain if the GOP retakes the White House.

With Reid’s retirement, Nevada has the only Democratic-held seat in jeopardy in 2016, and if Democrats can retain it, a lot of pressure would be placed on the five freshman Republicans who are seeking reelection in states that President Obama won twice: Illinois, New Hampshire, Ohio, Pennsylvania and Wisconsin.

Replacing Reid on the ballot will be Catherine Cortez Masto, a former state attorney general whose campaign will rely heavily on the political infrastructure the outgoing senator assembled in the past decade. The collection of labor unions, particularly casino workers, and burgeoning populations of Hispanics and Asian Americans have turned a state that traditionally tilted Republican in presidential elections toward Democrats in recent years.

The question is whether the machine can deliver the state’s electoral votes for the Democratic presidential nominee for a third straight election, while also retaining the Senate seat with Masto, a political figure who represents a sharp departure from Reid. There’s no doubt that Reid, 75, a Mormon with more than four decades of statewide campaigns under his belt, presents a different profile from Masto, 51, who is attempting to become the first Latina elected to the U.S. Senate.

“I’m younger and Latino with a different voice and different issues that we’ve worked on,” Masto said recently at the state party’s headquarters south of this city’s famed Strip.

Those differences became clear Monday, when Masto broke from Obama and congressional Democratic leaders to “share the concerns of some Republicans in Congress” about the vetting process for Syrian refugees, in light of Friday’s attacks in Paris and the potential connection of terrorists exploiting the refu­gee crisis to move into western Europe. In the interview, she signaled a more hawkish foreign policy than Obama or her mentor, Reid, supporting a more robust arming of rebels, such as the Kurds in Iraq.

That position will test whether this next-generation candidate can maintain the same support as Reid and Obama, while distancing herself from them on certain issues to demonstrate her independence.

Given the stakes, Republicans in Nevada are just as energized. A GOP win here by their likely candidate, Rep. Joe Heck, would probably ensure their majority for two more years. And there is no discounting the desire among some Republicans for the symbolic victory of grabbing the seat held by a political enemy for the past 12 years. Party leaders would like to play down that notion, however.

“I don’t think of it in those terms,” said Sen. Roger Wicker (Miss.), chairman of the National Republican Senatorial Committee. “I think it’s a great opportunity for a pickup. We’re looking at defending our majority and building on it. It’s a very important seat for that reason.”

The climate of the 2016 Senate race is relatively rare; Reid and Sen. Marco Rubio (R-Fla.), who is leaving the Senate to run for president, are the only two senators retiring from competitive swing seats. Most critical races will feature a first-term Republican trying to win reelection in a much different climate from the 2010 midterms, when the GOP seized the House majority and cut deep into Reid’s majority at the time.

To some degree, Reid is back where he started as the Democratic leader in 2005, serving in the minority trying to regain the majority while also trying to shore up his home flank.

He said that he is “devoting a huge amount of my time” to helping Masto win at home and to assure that his likely successor, Sen. Charles E. Schumer (N.Y.), takes the baton from Reid with a new majority.

“It’s so much better to be majority leader than minority leader,” Reid said at The Washington Post’s “2016 Pregame” event on the eve of last month’s presidential debate here.

Although Reid won reelection in 2004, President George W. Bush also was reelected and carried Nevada, the eighth Republican in the previous 10 presidential campaigns to do so. Republicans held a slight edge in voter registration at the time, and Reid realized he needed to rebuild the state’s Democratic Party. His idea was to push Nevada from the middle of the pack on the presidential primary and caucus calendar closer to the front of the line.

Officially, the effort was about placing a diverse western state and a heavily African American state, South Carolina, behind the small states of Iowa and New Hampshire. Those two traditional early-voting states did not, Reid explained, “demonstrate what America is all about, for a lot of different reasons. Nevada does that.”

Not only was Nevada early on the calendar in the 2008 contest, it became a brutal primary battle between Obama and then-Sen. Hillary Rodham Clinton (N.Y.), in which each candidate raced to register as many new voters as possible, particularly Latino workers in Las Vegas who had never been considered part of a crucial voting bloc.

By the end of the 2008 primary season, Democrats held a voter registration edge of more than 100,000 over Nevada Republicans, a stunning turnaround that played a key role in giving Obama a 12 percentage-point victory in the general election. Moreover, that effort, led by Reid’s top political adviser Rebecca Lambe, fortified his own advantage heading into the 2010 election, which Reid won handily.

In 2014, however, the state was less friendly toward Democrats. Republicans won the most prominent statewide offices and the GOP has cut the voter registration deficit to less than 60,000. If Heck becomes the Senate nominee, Republicans will have landed a candidate who will run more heavily on his biography than on his experience serving three terms in the House.

“A general, a doctor, small-business person, a great legislator, a good vote-getter in a swing district,” Wicker said, rattling off the résumé of the brigadier general in the Army Reserve. “I like our chances.”

Masto, however, intends to make the race very much about Heck’s record in Washington, particularly the positions he has taken on immigration. Hispanics now account for about 15 percent of the votes in Nevada — and nearly 30 percent of the entire population — so Heck’s support for revoking birthright citizenship for all immigrants will be a frequent refrain from Masto and her outside supporters.

Heck has opposed efforts to block Obama’s executive action that stopped the deportation of children brought to the United States illegally by adults, trying to walk a fine line between that and opposing other immigration-related executive orders.

She wants to focus the race on her record as attorney general of prosecuting sex traffickers and business fraud that led to the mortgage crisis. Republicans accuse her of marching to Reid’s orders in at least one instance, the prosecution of a Republican in a corruption case that was later dropped, when that GOP official was considering challenging Reid in 2010.

Back then, Reid benefited from a far-right conservative opponent, Sharron Angle, who won a heavily contested primary; Democrats are again trying to bait Angle into a primary campaign against Heck.

Two years later, in 2012, Sen. Dean Heller (R) edged his way past the Reid machine in a presidential election year, defeating Democrat Shelley Berkley by 1 percent. In that case, Berkley was battling ethics issues and Heller’s plurality victory, at 46 percent, was aided by an unusual state law that allows Nevadans to vote “none of the above.”

Ultimately, Masto is trying to thread the needle to run under the full auspices of that massive political infrastructure Reid built for Democrats, but also run as her own person.

“I’m a different voice,” she said.