WASHINGTON: The battle for control of the US Congress entered its final six months on Sunday, with midterm elections in November that could reshape President Donald Trump's second term and redefine the balance of power in Washington.

As ever, the midterms will be a referendum on the president, with Democrats hoping to capitalise on economic discontent and Trump's sliding approval ratings to retake the House of Representatives and Senate — while Republicans fight to defy political headwinds.

At stake is not just legislative control but the trajectory of Trump´s agenda, with a Democratic-run Congress able to launch investigations, block nominees and generally complicate the remainder of his presidency.

Early indicators point to a challenging environment for Republicans, with Trump's approval rating around 40% and economic dissatisfaction — particularly inflation and Iran war-related costs — eroding confidence.

Polling shows Democrats with a narrow edge on the generic ballot, while some surveys suggest voters now trust them more on the economy.

A Republican-aligned group, AFP Action, warned the party's Senate majority was "at risk," citing "structural headwinds" and disengaged voters.

In midterm elections, Americans vote to fill all 435 House seats, about one-third of the 100-member Senate and most governorships. Democrats need to flip three House seats and four in the Senate to take control.

"The Senate is on the table here," Molly Murphy, of pollster Impact Research, told cable network MS NOW, pointing to Trump's waning popularity and strong Democratic turnout in recent elections.

"When the president's approval rating is at or below 40%, that is when you start to see these deeper pickups in much more Republican-heavy terrain."

Still, the landscape remains fluid, with structural challenges for both parties.