Is GOP losing its grip on Ohio? Primary could be a signal of things to come.

Ohio’s March 19 primaries are grabbing national attention as Republicans look to maximize their shot at picking up congressional seats. Endorsements by Donald Trump and GOP Senator JD Vance have defined some races.

First Look

Five states will hold presidential primaries on March 19 as President Joe Biden and former President Donald Trump continue to lock up support around the country after becoming their parties’ presumptive nominees.

Mr. Trump is expected to easily win GOP primaries in Arizona, Florida, Illinois, Kansas, and Ohio. Mr. Biden is expected to do the same in all those states except Florida, where Democrats canceled their primary and opted to award all 224 of their delegates to Mr. Biden. That’s not an unusual move for a party with an incumbent in the White House seeking reelection.

Other races outside of the presidency could provide insight into the national political mood. Ohio’s Republican Senate primary pits Trump-backed businessman Bernie Moreno against two challengers, Ohio Secretary of State Frank Frank LaRose and Matt Dolan, whose family owns the Cleveland Guardians baseball team.

Chicago voters will decide whether to assess a one-time real estate tax to pay for new homeless services. And voters in California will move toward deciding a replacement for former House Speaker Kevin McCarthy, who resigned his seat after being pushed out of Republican leadership.

Mr. Trump and Mr. Biden have for weeks been focused on the general election, aiming their campaigns lately on states that could be competitive in November rather than merely those holding primaries.

Mr. Trump on Saturday rallied in Ohio, which has for several years been reliably Republican. But there are signs the state could be competitive again in 2024. Last year, Ohio voted overwhelmingly to protect abortion rights in its constitution and voted to legalize marijuana.

Mr. Biden, meanwhile, is set to visit Nevada and Arizona on March 19.

Closely watched Ohio GOP races

Republicans are watching a couple of high-profile elections in Ohio on March 19 that could determine their chances of picking up critical seats this fall and expanding their power in Washington.

One is a contentious and expensive Republican primary for the chance to face third-term Sen. Sherrod Brown this fall. The second is a GOP matchup in the 9th Congressional District held by Rep. Marcy Kaptur, of Toledo, the longest-serving woman in Congress.

Both Mr. Brown and Ms. Kaptur are considered among the year’s most vulnerable Democrats, amid Ohio’s tack to the political right in recent years. With Democrats holding a narrow voting majority in the Senate and Republicans maintaining a thin margin in the U.S. House, both races have already drawn outsized attention from national party leaders.

Two competitive districts, the 2nd and the 6th, lean conservative and are likely to go for Republican candidates in the fall. The 6th District includes the village of East Palestine, which is still suffering from the effects of a toxic train derailment last year.

The Democratic primaries, while competitive, promise to be less nail-biting than the GOP contests, which feature a congested race into which nearly a dozen contenders have poured a combined $4.5 million.  

Republican Rick Tsai, a chiropractor from East Palestine, is running on the derailment issue. But he’s in a tough fight against two current lawmakers, state Rep. Reggie Stoltzfus and state Sen. Michael Rulli, both of whom are better funded and have secured key endorsements, including those from East Palestine officials.

Eleven Republicans will have their names on the primary ballot for the 2nd Congressional District, which extends from the eastern suburbs of Cincinnati to the West Virginia border. The winner will face Democrat Samantha Meadows in the November general election.

Two of the better-known candidates are state Sens. Shane Wilkin and Niraj Antani. Political newcomers include businessmen Dave Taylor and Larry Kidd, as well as retired U.S. Marine drill instructor Tim O’Hara. Mr. Taylor raised $1.7 million for his campaign and Larry Kidd raised $1.4 million, while Mr. O’Hara raised $1.3 million.

All five are supporters of former Republican President Donald Trump and are running on conservative platforms such as supporting gun rights and opposing abortion.

Breaking from the pack is Phil Heimlich. Mr. Heimlich, son of the doctor who invented the Heimlich Maneuver, is a moderate Republican who does not support Mr. Trump, the presumptive Republican nominee in this year’s presidential election.

This story was reported by The Associated Press. AP writer Brian Slodysko in Washington contributed to this report.