Donald Trump’s selection of Indiana Gov. Mike Pence, a strait-laced and seasoned conservative, as his running mate Friday was designed to be a soothing overture that could repair the fractured Republican Party and signal a newfound discipline in the celebrity billionaire’s bid for the White House.

But Trump’s apparent 11th-hour indecision and private hesi­ta­tion about Pence, coupled with a delayed and fitful introduction, threatened to undercut part of the rationale for Pence joining the ticket: steadying a turbulent general-election campaign.

Trump announced Friday on Twitter that he had chosen Pence and that they would make their first joint appearance at a news conference Saturday in New York. The social-media proclamation capped a period of extraordinary uncertainty and mixed signals about the selection, just days before the Republican National Convention is set to open here in Cleveland.

“I’m very excited, very humbled and very grateful,” Pence told reporters gathered outside his Manhattan hotel as he made his way to Trump Tower for a 90-minute meeting with the candidate and campaign advisers.

In Pence, Trump has a classically credentialed if generic campaign partner. Trump, 70, will rely on the 57-year-old Midwesterner to shore up support where Pence has nurtured deep relationships, such as on the Christian right and with the conservative movement’s moneyed establishment. A former chairman of the House Republican Conference, the ideological purist was embraced by many corners of the Republican coalition Friday that had been cool to Trump’s candidacy.

The Fix’s Chris Cillizza explains why Indiana Gov. Mike Pence was the best vice-presidential pick among the candidates Donald Trump considered. (Peter Stevenson/The Washington Post)

But there were also immediate signs that Pence could shift the focus of the overall debate in ways Trump may not intend. Pence brings a visceral ideological edge to what has been a populist campaign centered on economic grievances and strident nationalism.

While Trump mostly avoids social issues on the campaign trail and his positions have evolved over the years, Pence has a history of vocally promoting a hard-line conservative agenda — from opposing same-sex marriage and abortion rights to defunding Planned Parenthood.

Democrats were swift to eviscerate Pence and portray him as divisive, intolerant and out of touch with the diversifying nation. Presumptive Democratic nominee Hillary Clinton’s campaign issued a video attacking Pence on issues related to women’s health, gay rights and immigration. The video ends with this message: “Donald Trump and Mike Pence: building a great, big, beautiful wall between America and progress.”

Trump’s campaign, meanwhile, left Pence largely defenseless. After Trump’s tweet, the campaign did not distribute a video or other promotional materials to relate Pence’s life story and governing accomplishments, nor did it forcefully push back against the Democratic attacks.

Pence emerged from an intensive vetting process that yielded two other finalists, New Jersey Gov. Chris Christie and former House speaker Newt Gingrich. Trump agonized over the most consequential choice of his campaign so far.

Throughout the day Thursday, his campaign sent strong signals that Pence would be the ­vice-presidential pick — even though Trump, fundraising in California without his closest advisers at his side, had not formalized his decision and postponed a planned Friday rollout, citing the terrorist attack in Nice, France.

Presumptive Republican presidential nominee Donald Trump picked Indiana Gov. Mike Pence (R) as his running mate. Here’s what you need to know about Pence. (Peter Stevenson,Danielle Kunitz,Osman Malik/The Washington Post)

In a string of media interviews Thursday night, Trump insisted that he had not made “a final, final decision” and praised all three finalists equally.

Privately, he deliberated late into the night with his top aides and family — torn between his pull toward Christie, a loyal soldier and charismatic political fighter, and Pence, whom advisers convinced him was the safer and more astute prospect, according to three Republicans with knowledge of the deliberations, who spoke on the condition of anonymity because the discussions were confidential.

Meanwhile, Pence had flown to New York that afternoon in advance of the planned Friday event. Back in Indianapolis, his allies prepared for him to withdraw from his gubernatorial reelection campaign. Pence faced a deadline of noon Friday to file papers with the state taking his name off the ballot for governor.

By the time Pence flew to New York, he felt “reassured. It was never not him,” according to one of the Republicans briefed on the deliberations. Paul Manafort, the Trump campaign chairman, kept in close touch with Pence and his aides throughout the day and night to help soothe any nerves.

Manafort denied that he had to persuade a hesitating Trump to formalize his offer to Pence. “Not true at all,” he said in an email.

Still, many Trump associates said Manafort was the linchpin behind Pence’s selection, arguing forcefully that it would pay political dividends for Trump, even though the two men barely knew each other and Trump harbored doubts about Pence’s talent as a campaigner and media combatant.

Gingrich applauded Pence in an interview Friday, calling him a “very effective sidekick for Donald Trump.” He said, an hour after the Pence announcement, that he had not heard directly from Trump about his decision but was “totally committed” to supporting the ticket.

“Mike Pence is a very good choice, and he has described himself as a Reagan-Kemp-Gingrich Republican,” Gingrich said.

Kellyanne Conway, a Trump strategist who is also a longtime Pence adviser, said the two will “complement each other in tone and content and style.” She dismissed analysis that Pence’s folksy personality will not click with Trump’s brashness.

“Pence has a very latent and very robust sense of humor that will surface fairly quickly,” Conway said. She also described him as an attack dog ready to take on Clinton, only at “five or six decibels,” and said that his understated presence would “calm a lot of donors and voters.”

Clinton’s campaign signaled that it was ready for such a brawl. In a statement, campaign chairman John Podesta said that “Pence is the most extreme pick in a generation.”

Clinton backers criticized Pence as a social warrior. Chad Griffin, president of the Human Rights Campaign, a leading gay rights advocacy group, called him “the face of anti-LGBTQ hate in America.”

The governor, Griffin said, “has made attacking the rights and dignity of LGBT people a cornerstone of his political career — not just a part, but a defining part of his career.”

Pence’s gubernatorial tenure has been marked by a law he signed last year that could have allowed businesses to refuse service to gay people — sparking a national firestorm and a backlash from the business and ­professional-sports communities that forced Pence to revise the statute.

Pence has not always agreed with Trump’s policy ideas. He has criticized Trump’s proposed temporary ban on Muslims entering the United States and long has been a proponent of trade, voting for every free-trade agreement during his six terms in Congress.

Like Trump, Pence has a history of disobeying the wishes of his party’s leadership. In Congress, he voted against three top priorities of President George W. Bush: the No Child Left Behind education legislation, Medicare Part D and the 2008 bank bailouts. All three votes would endear him to grass-roots conservative activists, and he became a popular speaker and fiery critic of President Obama at early tea party rallies.

Pence twice weighed running for president, ahead of 2012 and in 2016, both times opting instead to focus his ambitions back home in Indiana.

A native of Columbus, Ind., and raised by Irish Catholic Democrats, Pence graduated from Hanover College and later earned a law degree from Indiana University. He and his wife, Karen, an educator and watercolor painter, live in Indianapolis. They have three children: Michael, 24, who serves in the Marine Corps; Charlotte, 22, a filmmaker; and Audrey, 21, a student at Hanover College.

During his 12 years in Congress, Pence rose to relative prominence with frequent appearances on Fox News and talk radio. But his mark inside the Capitol was less noticeable. Despite his leadership titles — he chaired the House Republican Conference and, before that, the Republican Study Committee — he rarely participated in crafting major legislation.

Trump’s selection of Pence met with approval on Capitol Hill. House Speaker Paul D. Ryan (R-Wis.), who has offered only tepid support for Trump, said he will “do everything I can” to help the ticket win, noting his long friendship with Pence.

“Mike Pence comes from the heart of the conservative movement — and the heart of America,” Ryan said in a statement. “I can think of no better choice for our vice-presidential candidate.”

Jenna Johnson in New York and Sean Sullivan in Washington contributed to this report.