In theory, Sen. Timothy M. Kaine of Virginia is Hillary Clinton’s ideal wingman.

A scandal-free former governor from a crucial swing state, Kaine would be a genial antidote for a Democratic presidential nominee tarred by decades of controversy.

Even Republicans struggle for ways to attack Kaine, a Harvard Law School graduate who is fluent in Spanish and who volunteered as a missionary in Honduras.

As he emerges as Clinton’s potential running mate, Kaine’s tenure in public office — lauded by Republicans and Democrats alike — is evidence that nice guys can finish first in American politics.

But 2016 is not the year of the nice guy.

Sen. Tim Kaine (D-Va.) will campaign Thursday in Northern Virginia with Democratic presidential front-runner Hillary Clinton, who is considering Kaine as a running mate. (Molly Riley/AP)

In the most hard-bitten, vitriolic presidential campaign in generations, it is far from certain that Kaine would be ferocious enough to combat Donald Trump, the presumptive Republican nominee who tramples all manner of political decorum.

Speculation about a Clinton-Kaine ticket gained momentum Thursday when the senator gave a bilingual welcome to Clinton at a rally in Annandale, in Northern Virginia.

“Hillary is ready to make history,” Kaine said with an intensity that was more Bunsen burner than bonfire.

The joint appearance was a chance for Clinton to audition a potential running mate from a pool that includes Sen. Elizabeth Warren (D-Mass) and Labor Secretary Thomas Perez.

It was also an opportunity for voters across the country to take the measure of Kaine, whose unabashed earnestness can startle even the most jaded Washington observers.

“The boy is cleaner than the board of health,” said Dave “Mudcat” Saunders, a Democratic political consultant who says he is voting for Trump and whose clients have included Sen. Mark R. Warner (D-Va.) and the 2008 presidential campaign of then-Sen. John Edwards (D-N.C.). “If it’s one thing Hillary needs, it’s clean.”

A pragmatist with proven political savvy, Kaine had the foresight to endorse then-Sen. Barack Obama’s 2008 presidential campaign more than a year before his nomination.

Kaine distinguished himself in Virginia a decade ago, seizing on Virginia’s seismic demographic shifts to defeat a Republican and become governor.

Kaine, 58, who declined through a spokeswoman to be interviewed for this article, is not without potential vulnerabilities.

As governor and lieutenant governor, Kaine accepted $160,000 in gifts, all of them legal under Virginia law and all of which he declared in public disclosure statements. About three-fourths of the gifts were Kaine’s work-related travel expenses, according to his spokeswoman, including about $45,000 to attend Obama campaign events in 2008.

The gifts also included Kaine’s week-long use of a friend’s Caribbean vacation home in 2005, valued at $18,000. He also accepted $5,500 in clothes from a friend who owned a chain of clothing stores.

Although his spokeswoman, Amy Dudley, said Kaine “went beyond the requirements” of Virginia’s disclosure laws, Republicans are eyeing the gifts as a way to sully the senator.

Chris LaCivita, a GOP consultant, said that Kaine, who was elected to the Senate in 2012, is unknown beyond Virginia, a fact that will make it easier for Republicans to define him to a national audience.

“A lot of people are going to go ‘Tim who?,’ and anytime you have a blank canvas in politics, you have a lot of people standing around with paintbrushes,” La­Civita said. “Tim Kaine will not take the negatives off Hillary Clinton. He will incorporate hers.

“They run as a team. They run as one.”

At the same time, Republicans acknowledge that Kaine is not an easy target. “It’s hard to find any flies on Tim Kaine,” said former congressman Thomas M. Davis III (R-Va.). “He’s a thoroughly honest and decent man.”

On Wednesday, GOP officials in Virginia hosted a telephone call for reporters during which they planned to criticize Clinton and Kaine on the eve of their joint appearance.

After listening to the officials question Clinton’s character and intellect, a reporter asked about the senator.

Twelve seconds of silence elapsed before Rep. Bob Goodlatte (R-Va.) said, “I really don’t have any comments on Senator Kaine.”

John Whitbeck, the state GOP chairman, characterized as “very sad” that Kaine, with his “reputation for integrity,” would align with Clinton.

“He is going to be called on to defend the indefensible,” Whitbeck said.

Too ‘boring’?

A contemplative man, Kaine speaks in measured cadences, his round cheeks framing a cherubic smile.

His choice of words can be disarmingly retro. “Hey! What a treat!” he said when greeting a stranger recently.

Even Democratic partisans fret that the senator’s sunny, aw-shucks style may be unfit for a campaign dominated by anti-establishment bombast.

“Tim Kaine Is Too Boring to Be Clinton’s Running Mate” was the verdict rendered in a recent headline by the left-leaning New Republic magazine. Already disenchanted with Clinton, allies of Sen. Bernie Sanders (Vt.), her Democratic primary rival, worry that Kaine wouldn’t be able to rouse ardent progressives.

“If you want to inspire Sanders voters, he’s a zero,” said Robert Borosage, co-director of the Campaign for America’s Future, a Washington-based progressive advocacy group. “If you want to ensure that the African American vote turns out, he’s a zero. If you want to make a boring, safe choice, you can choose him, but he’s not going to turn out anyone to vote.”

Yet Kaine’s affability should not be confused for a bland biography. In the case of African Americans, for example, Kaine forged a strong bond with Richmond’s black community by joining a predominantly black Catholic church and eventually becoming mayor of the majority-black city.

Richard Harpootlian, a Sanders supporter and former chairman of South Carolina’s Democratic Party, said Kaine “could get folks like me excited about the Clinton campaign.”

“If Tim Kaine were the vice-presidential candidate,” he said, “I’m someone who would knock on doors and write checks.”

During a recent appearance on NBC’s “Meet the Press,” Kaine acknowledged his own lack of sizzle, declaring, “I am boring.” But he added jokingly, “Boring is the fastest-growing demographic in this country.”

A week later, the senator demonstrated his studiousness when he hosted a roundtable on the Zika virus in Richmond, summarizing the crisis for 15 minutes without referring to notes.

“I’m particularly interested in vector control solutions,” Kaine told a panel of medical experts, one of several moments when he easily slipped into the jargon of disease prevention.

At the roundtable’s conclusion, he took nearly a dozen questions from a cluster of reporters, more than half of which concerned his potential candidacy for vice president.

“The only role I’m playing is we’re trying to help her win Virginia,” he said of Clinton, his grin in place as he repeated an answer he had just given to another question.

A spiritual focus

On his way to the exit after the Zika event, Kaine interrupted a reporter who began to ask how he felt being considered for the vice presidency after starting in relatively humble Richmond city politics.

“Speculation is nice, but I’m a local guy,” Kaine said. “If I’m good at anything in politics, it’s because I learned it being a city councilman on the Richmond City Council.”

The senator’s humility is rooted in a childhood spent in the suburbs of Kansas City, Mo., as the oldest of three boys whose father owned an iron-welding shop.

His Catholic parents were so devout, the senator joked during a recent C-SPAN interview, that “if we got back from a vacation on a Sunday night at 7:30 p.m., they would know the one church in Kansas City that had an 8 p.m. Mass that we can make.”

By his own account, Kaine was a “nerdly student” in high school, after which he attended the University of Missouri. He majored in journalism until he decided the budding reporters were “too cynical” and switched to economics.

At Harvard Law School, Kaine took a year off to work as a missionary in Honduras, where he ran a program teaching carpentry and welding. “I do what I do for spiritual reasons,” Kaine said during the C-SPAN interview. “I’m always thinking about the momentary reality but also how it connects with bigger matters of what’s important in life.”

At Harvard, Kaine met his future wife, Anne Holton, whose father, A. Linwood Holton Jr., had been a Republican governor of Virginia. As governor, Holton helped integrate Richmond public schools by sending his children to all-black schools.

After law school, the couple moved to Richmond, where Kaine worked as a civil rights lawyer specializing in housing discrimination. In 1994, he won a seat on Richmond’s City Council, whose majority-black members selected him as mayor four years later.

In 2001, Kaine was elected lieutenant governor. (Warner was elected governor.) Four years later, Kaine became governor. During the campaign, he broke with political tradition by largely ignoring the state’s conservative rural outposts to focus on the Northern Virginia suburbs, where his fluency in Spanish helped him bond with an exploding Latino population.

An opponent of the death penalty, Kaine was able to convince conservative voters that he would not allow his personal beliefs to interfere with the state’s law. He also convinced abortion rights advocates that he would not impede on a woman’s right to choose despite his personal opposition to abortion.

Just after he became governor, Democratic leaders in Washington confirmed Kaine’s status as a rising star by drafting him to respond to then-President George W. Bush’s State of the Union speech.

Kaine’s remarks were sufficiently wooden that Jon Stewart of “The Daily Show” said they “lacked passion, insight or any sign of carbon-based life.” Others poked fun at his left eyebrow, which floated incongruously high on his forehead as he spoke. “Tim Kaine Eyebrow Watch: Still Arched, Weeks Later,” wrote Wonkette, an online blogger.

Increasing influence

Nevertheless, Kaine’s status among Democrats grew.

He was a finalist in the 2008 vice-presidential sweepstakes. Kaine was “really well liked by Senator Obama and all of us,” said David Axelrod, a senior Obama adviser at the time. “But he was only halfway through his term as governor at that point and had little foreign policy experience.”

Because of term limits, Kaine left the governor’s mansion in 2009 with few signature achievements, in part because of partisan gridlock. “You ended up with a governorship that didn’t have huge policy legacies,” said Stephen Farnsworth, a political science professor at the University of Mary Washington.

Obama persuaded Kaine to lead the Democratic National Committee in 2009, after which Democrats lost several key statewide races. Because of the defeats, L. Douglas Wilder, a former governor of Virginia, urged Obama to fire Kaine as DNC chair.

Kaine remained in his job but announced the following year that he was running for the Senate seat relinquished by Democrat Jim Webb. After defeating Republican George Allen, Kaine became the first senator to deliver a speech in Spanish from the floor of the Senate. His cause was immigration reform.

Sen. Angus King (I-Maine) said Kaine has impressed him for having “the ability to work with people without sending up partisan flags.”

King thinks Kaine would make not only an excellent vice president but also president — a sentiment he has shared with his colleague.

“I told him I’d manage his campaign,” said King.

He added that his friend’s understated response was not altogether unexpected. “He rolled his eyes.”

Jenna Portnoy contributed to this report.