WASHINGTON — Voters throughout much of the country will cast ballots for their U.S. senator this fall, but just nine states hold the power to determine control of the chamber — setting the course for which presidential nominees can secure confirmation and what bills advance during the next two years.

The Senate races, as well as the campaigns for every House seat, will serve as a test of Republican policies after two years of unified control of Congress. The outcome of this year’s midterm elections will also have a substantial impact on the remainder of President Donald Trump’s term.

Republicans hold 53 Senate seats, requiring Democratic candidates to win over voters in at least four states that previously chose a GOP lawmaker.

Zachary Peskowitz, political science professor at Emory University, said Republicans have a bit of an advantage this year, but that doesn’t guarantee they’ll keep their majority.

“I think that the Senate is a tougher map for the Democrats than the House is,” he said. “But given Trump’s approval ratings, obviously it is certainly possible that Democrats win, even in these tougher states.”

Alaska, Iowa, Maine, North Carolina, Ohio and Texas could be won by either candidate, according to a top elections forecaster, giving Democrats several paths to regain the Senate. That marks an improvement in the party’s chances, though they may also need to secure wins in competitive races in Georgia, Michigan and New Hampshire.

A 50-50 split would keep Republicans in charge of the Senate with the vice president casting the tie-breaking vote.

While each Senate candidate has a different message and variations in their policy goals, big-picture issues like the war in Iran, immigration enforcement tactics and the rising cost of living are at the top of voters’ minds. That’s been clear on the campaign trail.

So far, polling and analysis haven’t indicated a clear outcome to this fall’s elections.

The Cook Political Report with Amy Walter rates Alaska, Iowa, Maine, Michigan, Ohio and Texas as toss-up races. Campaigns for Senate seats in Georgia, North Carolina and New Hampshire currently lean toward Democrats.

Iowa and Texas had been in the lean Republican category but CPR changed that in late August, with Senate and Governors Editor Jessica Taylor writing “we see the overall battle for the Senate as a toss up, but one in which Republicans may still hold the slimmest of advantages.”

“Even though, on paper, it should be extraordinarily difficult for Democrats to flip seats that President Donald Trump won by more than 13 points, it’s increasingly hard to ignore both public and private polling showing both races to be more competitive than the Lean category reflects,” she added.

Ratings could continue to fluctuate, possibly several times, before voting ends on Nov. 3, but the top concerns aren’t likely to change much during the next couple of months.

A Pew Research Center poll released in late July shows voters consistently want politicians to talk about the economy, with 29 percent saying it’s an important issue, followed by 9 percent focused on ethics and reform, 7 percent on immigration, 5 percent on health care, 4 percent on foreign policy and 4 percent on the president.