Senior Democrats in the House of Representatives said on Thursday they would insist President Barack Obama provide hard evidence that proposed free trade deals will boost median U.S. incomes, laying out tough terms to support his trade agenda.

The demands, made just hours before Obama arrives here to address a House Democratic retreat, are part of a renewed focus by the party on middle-class economic issues.

Republicans have made free trade a top priority and have called on Obama to bring Democrats into line.

Obama's State of the Union address last week proposed shifting tax breaks from the wealthy to the middle class. He also called on Congress to give him "fast-track" authority to negotiate trade agreements, something many Democrats oppose, fearing American workers would lose more ground.

"Show me a trade deal that not only increases GDP (gross domestic product), but increases the average worker's monthly paycheck and I'll be for it," Representative Steve Israel told reporters on the sidelines of the retreat.

"That's the message. We're open to a trade deal, but it's got to increase median household income and not just GDP. And if the administration can get us there, it will pass with Democratic votes."

Representative Chris Van Hollen, the top Democrat on the House Budget Committee, said each trade deal will need to demonstrate stronger paychecks and wages for American workers, but it will be difficult to evaluate the 12-nation Trans-Pacific Partnership agreement now being negotiated.

Israel, who headed House Democratic campaign efforts during November elections that ceded Senate control and more House seats to Republicans, now has a senior role shaping the Democratic Party's message.

He said many middle class workers felt cheated by manufacturing job losses that resulted from the 1994 North American Free Trade Agreement, and remain worried the U.S.economy could quickly slip back into recession.

"In 1992, James Carville said 'it's the economy stupid,'" Israel said, referencing former President Bill Clinton's election strategist.

"In 2015, it's my economy stupid. It's my paycheck. It's my pay stub. It's my cost-of-living. It's all got to be personalized in terms of values."

Israel, who represents part of Long Island, New York, has opposed trade deals in the last decade with Panama, Colombia and South Korea.

In his address to the retreat on Thursday, Obama is expected to outline plans to ease spending constraints on military and domestic programs.