Representative Hakeem Jeffries stood inside a union hall in Pennsylvania’s Lehigh Valley on Sunday, under a banner that read “AFFORDABLE” in oversize red letters.

In case the message was not clear enough, the lectern the New York Democrat stood behind blared the same slogan — “Fighting for an Affordable America” — with the word “affordable” several sizes larger than the others.

Then Mr. Jeffries, who was here in one of the nation’s most competitive congressional districts to mark the final 100 days of a campaign he hopes will win his party the House majority and him the speakership, made sure nobody missed the pitch.

“House Democrats are fighting for an affordable America” Mr. Jeffries told the crowd of several dozen supporters gathered in Bethlehem. “House Democrats are fighting to lower the cost of groceries, gas and housing.”

It is a theme that Democrats are betting will rally voters behind them as they battle for control of Congress, and Mr. Jeffries has made it clear that he means to keep returning to it time and again as he works to persuade voters to reject Republicans in places like this one around the country.

“That American dream is fading,” Mr. Jeffries said. “And the leaders who should be fighting to keep the dream alive are putting it further out of reach.”

The final months of the midterm election cycle offer Democrats considerable opportunity but also growing uncertainty. The party needs to gain only a handful of seats to win the House. Public polls show Democrats with an edge when it comes to voters’ preference for Congress, and Democratic candidates in key races have a slight financial advantage.

But the map leaves little room for error. And a succession of progressive primary victories over tenured Democrats more closely aligned with leadership has raised fresh questions about the makeup and unity of any majority Mr. Jeffries can manage to assemble.

With members of House back in their home states for a five-week summer recess, Mr. Jeffries’s appearance in Bethlehem was the opening stop of what he hopes will be an aggressive closing push through the districts that will decide control of the House.

“This is no longer a marathon,” he told the crowd of supporters. “It’s a sprint.”

According to figures provided to The New York Times by Mr. Jeffries’s team, he has helped to raise $137 million for House Democrats during the second quarter, including money to protect vulnerable incumbents and to boost candidates in their “Red to Blue” program targeting seats they see as crucial flips to claim the majority.

He was here in Pennsylvania with Bob Brooks, a firefighter and union leader who is challenging Representative Ryan Mackenzie, a first-term Republican who narrowly won the seat in 2024. Kicking off the final sprint in the state underscored its importance in Mr. Jeffries’s push for the majority; Democrats are also looking to flip three other congressional districts in Pennsylvania now held by Republicans.

Later this week, the leader is scheduled to headline events in New York, with stops including Buffalo and Rochester, before traveling to Southern California to continue fund-raising for Democratic candidates.

Mr. Brooks presented himself on Sunday as a political outsider whose own life reflected the economic pressures that Democrats have made central to the campaign. He recalled working as a paperboy, pizza delivery driver, dishwasher and bartender, and made the case to the crowd that he was best to address the economic pressures voters face.

“I understand them because I’ve lived them,” Mr. Brooks said to nods of approval. “I’ve lived in public housing. I’ve relied on food stamps when times were hard. I’ve had to choose between paying the medical bills and paying the mortgage.”

Democrats this cycle have increasingly sought out candidates with hardscrabble local roots outside of politics, and see Mr. Brooks and others like him as compelling messengers for a campaign cycle they hope will revolve less around ideological disputes than kitchen table issues.

But as Mr. Jeffries has urged candidates to rally around a pitch tightly focused on lowering the cost of living, a new generation of progressive Democrats has been toppling longtime incumbents in safe Democratic districts with more provocative messages, including calls for ending U.S. aid to Israel, abolishing Immigration and Customs Enforcement and getting rid of prisons.

In addition to Mr. Mackenzie here in Pennsylvania’s Seventh Congressional District, Democrats are targeting seats in other parts of the state held by Representatives Brian Fitzpatrick, Rob Bresnahan and Scott Perry.

“Those four races hold the key to who is in the majority in the Congress of the United States,” said Gov. Josh Shapiro, who appeared alongside Mr. Jeffries and Mr. Brooks.

All three sought to tie the affordability message to the policies of President Trump and House Republicans. Mr. Shapiro argued that while Pennsylvania had cut taxes and expanded benefits under his administration, Republicans in Congress had erased those gains.

“Every time we put a buck back into somebody’s pocket, Donald Trump and Ryan Mackenzie take two bucks out the other side,” he said.

Mr. Mackenzie, who held campaign events in nearby Bethlehem Township on Sunday, called the affordability messaging from Democrats “laughable,” telling the NBC affiliate in Philadelphia that when Democrats had control, “they had inflation at record levels, everybody saw it. Everybody felt it in everything all across their lives.”

Mr. Shapiro, meanwhile, pointed to gasoline prices at a nearby station as a daily reminder of the costs of the war in Iran, which Democrats have increasingly linked to rising fuel prices.

“It’s a reminder every day when you go to the pump, when you’re at a Wawa,” he said.

Mr. Brooks made the same argument against his Republican opponent.

“He campaigned on a promise to lower costs and invest here at home,” Mr. Brooks said. “Instead, he’s voted for a cost-spiking war of choice in Iran that has made everything more expensive.”

In his remarks, Mr. Jeffries avoided the ideological debates that have consumed parts of his party in recent months. Earlier Sunday, pressed during an interview on CNN about the progressive primary victories and disagreements within Democratic ranks, he insisted that members of his party simply needed to concentrate on their common goal of lowering costs.

“What unites the party,” he said, “is a focus on us fighting for an affordable America.”