HARRISBURG, Pa. — As Pennsylvania state senators filed through the tight, wood-paneled corridor that leads out of the Senate chamber this week, John Yudichak turned into an equally tight staircase that no Democrat ever takes: down to the Senate Republican majority caucus room.

Mr. Yudichak will take those stairs a lot now that he has left the Democratic Party after two decades in the Legislature to become a registered independent who is joining the Senate’s Republican majority caucus.

And while Mr. Yudichak described his defection as an effort to empower the political middle, it reverberated swiftly in the Capitol and through the state’s politics as something else.

To many, it dealt a blow to Democratic hopes of capturing the chamber’s majority in next year’s election for the first time in nearly three decades.

“What I can say is, this absolutely takes away any chance for Senate Democrats to gain control of the Senate chamber,” said the Senate President Pro Tempore Joe Scarnati, R-Jefferson.

To others, Mr. Yudichak’s defection underscored the political realignment in Pennsylvania accelerated by Donald Trump’s stunning 2016 win in the presidential battleground state.

Mr. Yudichak’s own Luzerne County was the poster child for Mr. Trump’s victory in Pennsylvania, when the county and the state backed a Republican presidential candidate for the first time since 1988.

Mr. Yudichak, 49, a moderate pro-labor descendant of unionized coal miners, is now the Senate’s only registered independent ever on record, Senate officials say.

Earlier in the year, Mr. Yudichak began talking to a friend and contemporary, Senate Majority Leader Jake Corman, R-Centre, about his career and, in September, floated the idea of becoming an independent and joining the Republican caucus.

“It was his suggestion, he asked,” Mr. Corman said. “He said, ‘You know, if I switched to an independent, would I be welcome in your caucus?’ and I said, ‘Of course you would be.’”

In making his announcement early last week, Mr. Yudichak blamed a corrosive “Us vs. Them” political world and maintained that his move wasn’t personal or about a single event.

Still, he said he would continue to support and fundraise for Democrat Joe Biden in the 2020 race for president, as well as Democratic candidates whose interests align with his.

But he characterized the Democratic Party and the Senate’s Democratic caucus as increasingly less tolerant of issues that are important to him.

Many inside the Democratic caucus accused Mr. Yudichak of betraying his roots.

“Something else is going on here,” said Sen. Vincent Hughes, D-Philadelphia. “I don’t know what it is, but it is going on.”

Perhaps for Mr. Yudichak the writing was on the wall.

As a northeastern Pennsylvania product, Mr. Yudichak had no prospect at becoming the Democrats’ floor leader in a caucus with a decidedly more liberal powerbase from Philadelphia and its suburbs.

Then there’s Mr. Yudichak’s district.

It has a solid Democratic registration advantage, but it bears the hallmarks of Trump country: it is whiter, with lower median incomes and lower rates of college-degree attainment, than the rest of Pennsylvania.

Many Democrats saw cold political calculation in Mr. Yudichak’s decision.

Sen. John Blake, D-Lackawanna, called it a “a selfish political bait-and-switch.”

“I understand the calculus of it and I understand John to be an ambitious politician, so maybe he’s trying to set himself up for something in the future,” Mr. Blake said.

Luzerne County’s Democratic Party chairman John Pekarovsky, who is also Mr. Yudichak’s campaign treasurer, was stunned. But, he said, he saw no selfish political motive for a politician who didn’t even have an opponent in his 2014 and 2018 re-election bids.

“John is so popular in our area,” Mr. Pekarovsky said.

Including Mr. Yudichak, Republicans are counting on a 29-21 Senate majority going into next year’s election, when Senate Democrats hope to harness an anti-Trump backlash in heavily populated suburbs.

Democrats now must capture four seats — instead of three — to capture a majority with Democratic Lt. Gov. John Fetterman as the tie-breaking vote.

That means Democrats are moving a Lancaster-based seat into the must-win category along with seats anchored in Erie, Harrisburg and Delaware County.

Democrats may even get help from Mr. Yudichak.

On Monday, the day before Mr. Yudichak announced he was leaving the Democratic Party, he asked Mr. Pekarovsky to cut two campaign checks.

Both were for Senate Democrats running for re-election next year, including one — Jim Brewster, D-McKeesport — who Republicans are gunning for in the 45th district.

First Published: November 23, 2019, 4:31 p.m.