WASHINGTON — There’s only so much that President Barack Obama can do to help his party this year. Six years in, he’s less popular than before, and aside from fundraising, his value on the campaign trail is limited, especially in the places that matter most for Democrats.

But the president can set the tone — and in his State of the Union address on Tuesday, Obama will deliver the opening salvo in a yearlong fight for control of Congress. Although not explicitly political, the speech before millions will frame an economic argument that Democrats hope will resonate with voters in races across the country.

“It will be interpreted as the Democratic agenda,” said Celinda Lake, a Democratic pollster. “He can frame up the 2014 choice.”

That choice, as Obama portrays it, is between an America where all segments of the population have opportunities to improve their lot and one where prosperity is disproportionately enjoyed by a select few. In the run-up to the State of the Union, Obama has persistently sought to focus the nation’s attention on trends of inequality and lower social mobility that he’s pledging to address in his final years in office.

To be sure, not every Democrat will echo Obama’s themes in their own campaigns. Many may focus on niche, regional issues or their personal characteristics. But with the economy still a top issue for most voters, Democrats see issues of economic fairness and expanding access to the middle class as their best chance to reach a broad swath of the population that feels left behind by the sluggish economic recovery.