Washington DC – Republicans finally rallied behind a new leader on Wednesday as the US House of Representatives finally elected its 56th speaker – ending weeks of party infighting that paralyzed Congress in a period of international and domestic crisis.

Louisiana's Mike Johnson, a staunch ally of Donald Trump and who spearheaded legal efforts to overturn the 2020 election, won unanimous support from his party to lead the lower chamber of Congress and draw a line under the chaos.

"It is the honor of a lifetime to have been elected the 56th Speaker of the House," he wrote on X after the vote made things official.

"It has been an arduous few weeks, and a reminder that the House is as complicated and diverse as the people we represent."

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"Our House Republican Conference is united, and eager to work," he added. "As Speaker, I will ensure the House delivers results and inspires change for the American people. We will restore trust in this body."

The Louisiana congressman is the fourth Republican picked by his party in three weeks to replace the ousted Kevin McCarthy, and lawmakers are cautiously optimistic that he can succeed where the others fell short.

Like his three colleagues, Johnson was nominated in an internal party ballot – but did not face the bitter opposition that stymied the others, who dropped out one-by-one as they failed to muster enough support to win on the House floor.

"This House Republican majority is united," Johnson told reporters as colleagues cheered and applauded following his nomination.

A virtual unknown, Johnson's lack of star power may help the 51-year-old attorney and religious rights campaigner, as he lacks enemies on his own side.

A junior member of the leadership as the conference vice chairman, Johnson entered the House of Representatives in 2017, having stirred controversy with legislation seen as anti-gay in Louisiana state politics. The married father-of-four also voted against codifying federal protections for same-sex marriage last year.

He was the ringleader among more than 100 Republicans who signed onto a legal brief supporting a lawsuit seeking to overturn the 2020 election results in four swing states won by President Joe Biden.

ABC News asked Johnson late Tuesday about the filing, which the Supreme Court declined to adjudicate, but he merely replied "next question" as his colleagues began jeering.

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The son of a fireman and a conservative talk radio host, Johnson is backed by House Majority Leader Steve Scalise and Judiciary Committee chairman Jim Jordan, who both failed in their own speaker bids.

Johnson is the least experienced speaker in the post Civil War era, having never chaired a committee or held a senior leadership role. And he will almost immediately preside over a government shutdown that could threaten his job unless he can cut a 2024 budget deal favorable to his party with more seasoned negotiators such as Senate Majority Leader Chuck Schumer to Biden.

He will also be expected to lead his deeply fractured conference through upcoming fights over funding for Ukraine and Israel in their conflicts with Russia and Hamas.

"We will restore sanity to a government desperately in need of it," Johnson wrote. "Let’s get back to work."