Former President Donald Trump’s pick for the Republican nomination for Pennsylvania’s open Senate seat suspended his campaign after losing his bid for custody of his three children amid allegations of abuse in a contentious divorce case.

Sean Parnell, an author and former Army Ranger, said he plans on asking the court to reconsider the decision, but he can’t continue with a Senate campaign.

“I...

Former President Donald Trump’s pick for the Republican nomination for Pennsylvania’s open Senate seat suspended his campaign after losing his bid for custody of his three children amid allegations of abuse in a contentious divorce case.

Sean Parnell, an author and former Army Ranger, said he plans on asking the court to reconsider the decision, but he can’t continue with a Senate campaign.

“I strongly disagree with the ruling today and I’m devastated by the decision,” Mr. Parnell said. “My focus right now is 100% on my children, and I want them to know I do not have any other priorities and will never stop fighting for them.”

Mr. Trump and his team put no pressure on Mr. Parnell to quit the race for the crucial battleground seat, according to aides for both men, and the former president wasn’t expected to make an immediate endorsement of another candidate, according to people familiar with his thinking. Mr. Trump and his team planned to wait and see whether any other candidates announced campaigns or how Mr. Parnell’s departure otherwise changed the dynamics of the race, these people said.

Among the Republicans already competing in the primary are Carla Sands, Mr. Trump’s former ambassador to Denmark, and Republican businessman Jeff Bartos. David McCormick, a Wall Street executive married to Dina Powell, Mr. Trump’s former deputy national security adviser, has been considering a possible Senate run. Dr. Mehmet Oz, the celebrity surgeon, also is reportedly weighing a campaign.

Taylor Budowich, a spokesman for Mr. Trump, said the state’s Senate race remains a priority for the former president. “Rallying our movement behind the best America 1st candidate remains critical,” Mr. Budowich posted on Twitter.

In an order made public on Monday, Judge James Arner awarded primary legal custody to Mr. Parnell’s estranged wife, Laurie Snell, saying she was “the more credible witness” and that Mr. Parnell had committed “some acts of abuse in the past.”

Ms. Snell had testified under oath that Mr. Parnell physically abused her and their children. Mr. Parnell denied the allegations in public statements and in sworn testimony.

The situation worried some Republican strategists working to win back the Senate majority, since Mr. Trump’s backing had given Mr. Parnell an edge in Pennsylvania’s hotly contested GOP primary, where multiple candidates are vying to succeed retiring GOP Sen. Pat Toomey.

Multiple candidates are vying to succeed retiring Republican Sen. Pat Toomey, who spoke during a Senate hearing last week in Washington.

Photo: Stefani Reynolds/Bloomberg News

President Biden narrowly won the state in 2020, and the Senate candidate who prevails there in next year’s midterm elections could determine control of the 50-50 Senate.

“Today’s developments will only throw Pennsylvania’s Republican Senate primary into further chaos,” said Pennsylvania Democratic Party spokesman Jack Doyle.

Democrats also have a competitive primary contest under way, with a field that includes Lt. Gov. John Fetterman and Rep. Conor Lamb.

Mr. Parnell, who narrowly lost to Mr. Lamb in a House race last year, announced he would run for Senate in May, and Donald Trump Jr.

quickly tweeted his support: “Sean is a friend and a strong conservative fighter with a spine of steel.”

In September, the elder Trump endorsed Mr. Parnell as well. “Sean bravely fought for our Country as a Captain in the U.S. Army and was awarded two Bronze Stars (one for valor!) and the Purple Heart,” the former president said at the time. “Unlike our current administration, he never left anyone behind.”

Write to Lindsay Wise at lindsay.wise@wsj.com and Michael C. Bender at Mike.Bender@wsj.com