Representative Steve King, who last year defended white nationalism, faces the toughest Republican primary challenge of his 17-year congressional career.

Theresa Greenfield is the choice of establishment Democrats, but she still faces tough competition in the race to challenge Senator Joni Ernst.

The contest to face Representative Xochitl Torres Small, a Democrat who flipped this seat in 2018, has been ugly. Claire Chase, an energy industry lobbyist, and Yvette Herrell, a former state representative who lost to Ms. Torres Small in 2018, have accused each other of being insufficiently supportive of President Trump.

Valerie Plame, the former C.I.A. agent, jumped into the race for this safe Democratic seat with a splash, but has proven to be an unsteady campaigner. Teresa Leger Fernandez, a lawyer, has emphasized her local roots and ability to speak fluent Spanish.

Democrats have a shot at flipping this seat, currently held by Brian Fitzpatrick in a district Hillary Clinton carried in 2016. But the party’s preferred candidate, Debbie Wachspress, dropped out in February.

Republicans are choosing a challenger to Susan Wild, a freshman Democrat, in this tossup district.

Republicans are competing for the chance to go up against Matt Cartwright, a Democratic incumbent in a district President Trump won by 10 points.

Scott Perry, the Republican incumbent, has one of the most conservative voting records in the House. He won his redrawn district in 2018 by less than three percentage points.

Representative Kweisi Mfume, who won a special election in April, is running for a full term. He faces more than a dozen candidates, including Maya Rockeymoore Cummings, who is making a second effort to fill her late husband’s seat.

Former Mayor Sheila Dixon, who resigned in 2010 as part of a plea deal, is trying to return to office in a race that includes Mary Miller, a former investment executive and Treasury official, and the incumbent mayor, Bernard Young, who trails far behind.

Steve Bullock, the term-limited Democratic governor who briefly ran for president and is now running for Senate, has endorsed Mike Cooney, the lieutenant governor, in the race to replace him. Whitney Williams, a businesswoman, is backed by Emily’s List, a group that supports female candidates.

Greg Gianforte, Montana’s lone congressman, lost to Gov. Steve Bullock in the 2016 race, then won his House seat in a 2017 special election after he was charged with assaulting a reporter. He appears to be in a competitive race with Tim Fox, Montana’s attorney general.