LOS ANGELES — On Tuesday, voters in California’s 25th Congressional District will choose the successor to former Representative Katie Hill, and Wisconsin voters in the state’s Seventh District will look to fill the seat left by former Representative Sean Duffy.

Wisconsin’s special election to fill the House seat vacated last year by Mr. Duffy, a Republican, is not expected to be close, but is being viewed by both parties as an important indicator of enthusiasm.

Like Wisconsin’s contentious April 7 elections, the voting in the district, which covers parts of 26 counties in the state’s north, will be conducted both by mail and in person, with National Guard troops deployed to help operate some of the polling locations.

In 2016, the district backed Hillary Clinton, the Democratic presidential nominee, over Mr. Trump by seven points — and a Republican win would most likely prompt hand wringing among Democrats, who are counting on suburban voters to take back the White House.

Tom Tiffany, a Republican state senator from Hazelhurst, Wis., is widely expected to beat Tricia Zunker, a Democrat from Wausau, Wis., and a school board official, in the special election for Wisconsin’s Seventh District congressional seat.