Donald Trump and fellow Republicans assailed President Joe Biden on Saturday for what they called an assault on Christianity, after the US leader highlighted the Transgender Day of Visibility that this year coincides with Easter.

It was the latest skirmish in the culture war shaping the White House race, as Trump seeks support from religious conservatives while Biden stakes his claim as the standard-bearer for an inclusive America.

Biden on Friday issued a proclamation recognizing Transgender Day of Visibility, falling every year on March 31, which this time is also Easter Sunday.

In a statement slamming Biden's "blasphemous" declaration, Trump campaign spokeswoman Karoline Leavitt said the move was part of "the Biden Administration's years-long assault on the Christian faith."

"We call on Joe Biden's failing campaign and White House to issue an apology to the millions of Catholics and Christians across America who believe tomorrow is for one celebration only -- the resurrection of Jesus Christ."

Republican House speaker Mike Johnson said on X it was "outrageous and abhorrent" to proclaim, "Easter Sunday as 'Transgender Day."

"The American people are taking note," he added.

The White House snapped back with a statement pointing out that Biden, a devout Catholic, "stands for bringing people together and upholding the dignity and freedoms of every American."

"Sadly, it's unsurprising politicians are seeking to divide and weaken our country with cruel, hateful and dishonest rhetoric," spokesman Andrew Bates said in a statement.

"President Biden will never abuse his faith for political purposes or for profit," he added, an apparent dig at Trump who this week urged his supporters to buy the $59.99 "God Bless the USA Bible."

Trump and Republicans are seizing on conservatives' discomfort over transgender rights, as some states restrict gender-affirming care for minors and prohibit people from using bathrooms not in alignment with their sex at birth.

"We will ban men from participating in women's sports," Trump has declared at campaign rallies, as he highlights policy priorities for a second White House term.

The Easter kerfuffle came after Trump drew criticism for a video on his Truth Social platform that featured an image of Biden hog-tied, as if he were being kidnapped.

The video, posted Friday, shows two pickup trucks driving down a highway, each displaying pro-Trump signs and flags.

On the tailgate of the second truck is an image of a prone Biden tied with his hands behind him.

"This image from Donald Trump is the type of crap you post when you're calling for a bloodbath or when you tell the Proud Boys to 'stand back and stand by,' Biden spokesman Michael Tyler told AFP, referring to the far-right militia involved in the 6 January 2021 assault on the US Capitol by Trump supporters.

He went on: "Trump is regularly inciting political violence and it's time people take him seriously -- just ask the Capitol police officers who were attacked protecting our democracy on 6 January."

The Trump campaign was unapologetic.

"Democrats and crazed lunatics have not only called for despicable violence against President Trump and his family, they are actually weaponizing the justice system against him," spokesman Steven Cheung said.

Trump has a long history of using dark and combative language and posting provocative images.

Trump in December accused migrants of "poisoning the blood" of Americans.

In November, he lashed into his political rivals, vowing to "root out the communists, Marxists, fascists and the radical left thugs that live like vermin within the confines of our country."

That drew a response from Biden, who said Trump was using "language you heard in Nazi Germany."