Sen. Charles E. Schumer (D-N.Y.) leaves after an interview with The Washington Post on Oct. 10. (Gareth Smit/For The Washington Post)

Sen. Charles E. Schumer pulled up on a recent afternoon to the historic train station in this tiny Hudson Valley commuter town.

But instead of jumping out of the car to greet voters who might help him win his reelection race, the New York Democrat first tended to more crucial business: a phone conversation advising a Democratic Senate candidate whose victory would help make him Senate majority leader after November.

Once he emerged, Schumer worked his way to the train platform, where he stood with local leaders — mostly Republicans — to bemoan the safety records of transit agencies carrying hundreds of commuters daily into New York City.

“They’re not up to snuff,” Schumer barked in the direction of television cameras, adding later: “We get them a lot of federal money, and we can ask for good service in return.”

Heads nodded, and, later, local Republicans praised Schumer’s personal attention. But Schumer had already said his goodbyes — he was back in the parking lot, chatting on his antiquated flip phone with Russ Feingold, the Democrat favored to reclaim his old Wisconsin Senate seat.

Sen. Charles E. Schumer (D-N.Y.) greets voters after a campaign event at the Hugh A Doyle Senior Center in New Rochelle, NY., on Oct. 10. (Gareth Smit/For The Washington Post)

Schumer is cruising to his own reelection to a fourth term this year against conservative activist Wendy Long. So he’s focusing on something bigger: the job of top dog in the Senate, majority leader, if Democrats can win the four seats it would take to get him there. He’s pouring much of his manic energy into that campaign by dispensing nonstop political advice to every Democrat running in a competitive Senate race.

Schumer, 65, is a force of nature. He estimates talking directly to at least six candidates a day. Katie McGinty, the Democratic nominee Schumer personally recruited into the race against Sen. Patrick J. Toomey (R-Pa.), says he is in “near constant” contact with her or her advisers on everything from overall campaign strategy to just bucking up her spirits.

Schumer is poised to become a critical player in Washington next year, particularly if Hillary Clinton wins the White House. During the duo’s eight years together as New York’s senators, they started as rivals but evolved into complementary partners. Now, the two have a symbiotic bond — Schumer’s success depends on how well Clinton does leading the ticket, possibly bringing along enough Democrats to seal the majority. Clinton’s success would then depend on her old colleague’s stewardship of the Senate.

He has been accused of putting his beliefs over broader Democratic goals, at times showing a willingness to challenge President Obama. He was deeply critical of the timing behind passage of the Affordable Care Act, the president’s signature domestic achievement, and opposed two of Obama’s foreign policy goals: the 2015 Iran nuclear deal and a bill passed this summer to allow victims of the 9/11 attacks to sue Saudia Arabia. On the latter, he helped lead the only veto override of Obama’s presidency, exposing an embarrassing intraparty feud.

Next year, Schumer will likely have to negotiate in a divided Washington with House Speaker Paul D. Ryan (R-Wis.), with whom he has sometimes worked behind the scenes.

It’s a tall order for anyone, but Schumer’s confidants say he is ready for the job.

“I call him the Jewish LBJ,” said Sen. Al Franken (D-Minn.), whose narrow 2008 victory was overseen largely by Schumer from afar.

Sen. Charles E. Schumer (D-N.Y.) pauses after a news conference on Oct. 10. at the Tuxedo Metro North Station in Hudson Valley, N.Y. (Gareth Smit/For The Washington Post)

Relatively unknown on the national stage, Schumer has become a lightning rod for Republicans trying to rally conservatives disillusioned with GOP presidential nominee Donald Trump.

“If Donald Trump becomes president, he can’t accomplish anything if Chuck Schumer’s in charge of the Senate. And if Hillary Clinton becomes president, you want to have a check on what could otherwise be a really, really radical administration,” Toomey told reporters Thursday.

It’s a sign of his respect inside the Democratic caucus that Schumer locked down unanimous support to succeed Minority Leader Harry M. Reid (Nev.) within a day of Reid’s announcement last year he would not seek reelection.

If he gets the majority, Schumer has set a goal that starts small to demonstrate that government can function. He wants to build off those early wins for the bigger prizes and would be the first Jewish floor leader of the upper chamber.

“It’s showing people that government can help them and be a force for good, which is what I want to try to do as majority leader. That, in a small way, is what I’m trying to do,” he said in a recent interview at a Manhattan diner.

Schumer remains the most prodigious fundraiser in the Senate, collecting roughly $25 million for his reelection race, where the only question is whether he can sweep all 62 New York counties. (He’s gone only 61 for 62 in previous races.)

He has taken his war chest and shifted $6.2 million to the Democratic Senatorial Campaign Committee (DSCC) and other groups allied with Democrats.

So now he spends his days traversing his state like he did in his first race but spending all his private time pushing for the majority. In a 40-minute drive from Long Island to the Hudson Valley, Schumer talked with the DSCC chairman, Sen. Jon Tester (D-Mont.), and two Democratic candidates, according to aides. From there to Westchester County, Schumer checked in with Tester’s top strategist and later dialed up Reid to update him.

Young Chuck Schumer’s first break was living in the same Brooklyn neighborhood as Stanley Kaplan, who founded the test-taking company. As a student at James Madison High School, Schumer worked the mimeograph machine in Kaplan’s basement, cranking out test-preparation booklets and bringing them home for his own studies. The result was an SAT score so high that the solidly middle-class Schumer glided into Harvard University. He went straight to Harvard Law and, after graduating in 1974, came home and won a seat in the New York State Assembly. He was 23.

Six years later, he won his first U.S. House race and moved into a Capitol Hill townhouse with several other young, ambitious lawmakers, including Leon Panetta, the future CIA director and secretary of defense. By 1994, Schumer was the lead sponsor of the Brady bill requiring background checks on gun purchases. He still vividly remembers a West Wing meeting where some of then-President Bill Clinton’s operatives worried about the bill’s political fallout, only to discover that Hillary Clinton, then first lady, was his chief defender. The law passed — and Democrats lost their majorities.

Tired of life in the House minority, Schumer knocked off Al D’Amato in a nasty 1998 Senate race, during which Clinton campaigned for him. By the time Clinton won her Senate seat two years later, Schumer longed for the attention that comes with being senior senator. But the big-city media were more enthralled with the first lady-turned-senator. The friction was palpable.

“We fought as a team. It took a while for us; we’re both Type-A personalities,” Schumer recalled in the interview, adding, “We had to learn — which we did in about a year — that working together was a lot better than working separately.”

Schumer toyed with running for governor, but after the 2004 elections, Reid made Schumer DSCC chairman — he steered the campaign committee with brutal effectiveness. Democrats went from 45 to 59 seats in his four-year term, including knocking off 11 Republican incumbents. Democrats secured a supermajority in early 2009, which paved the way for much of Obama’s agenda during his first two years.

In his DSCC days, Schumer worked with Trump, who then was a reliable Democratic donor. Trump hosted Schumer and other Democrats in February 2008 at his Mar-a-Lago resort for a big-dollar fundraiser — a slave to details, Schumer can still recall that the event netted $230,000 for Democrats.

Republicans don’t always think Schumer plays fair. Sen. Mitch McConnell (R-Ky.), then the minority leader, grew angry with Schumer in the fall of 2008 after they helped negotiate the bipartisan Wall Street bailout — after which Democrats ran ads attacking McConnell on the issue. Schumer’s position was that it was an independent committee that he could not control.

By 2012, Schumer was caught in the Senate gym talking on the phone with Elizabeth Warren about her race against then-Sen. Scott Brown (R-Mass.), according to those familiar with the two men. Brown’s account is that Schumer told her he would move votes around to prevent Brown from flying home to make a debate later that night. Schumer’s version is that he was only talking about the debate and the state of the race — and that Brown did in fact make it to the debate.

Schumer has worked to repair his relationship with McConnell.

“Will a reset help? I think it might,” Franken said. “I think Harry and Mitch have some history that sometimes can get in the way, so this might help.”

Schumer and McConnell have been shadow-boxing each other all year, each big move followed by a countermove. The majority leader talked Sen. Marco Rubio (R-Fla.) into running for reelection — three weeks later, Schumer persuaded former senator Evan Bayh (D-Ind.) to run for his old seat.

And now, in the campaign’s final days, Schumer finds himself reliant on Clinton, his onetime rival. She’s pulling away in Pennsylvania and keeping it competitive in red states such as Indiana. Some of Schumer’s closest aides work for Clinton’s campaign.

If Clinton and Schumer both win, then the really hard work of actually governing would begin.

“If we’re gridlocked for another four years, the anger and sourness in the land will make that of 2016 seem tame,” Schumer said. “So there’s a moral imperative to restore faith in government and restore faith in America.”