By MATTHEW KAMINSKI

West Palm Beach, Fla.

Marco Rubio is the newest Republican star, but not for any obvious reasons. On policy, the Senate hopeful from the Sunshine State blends in easily with the tea party crowd. Less government, taxes and debt, check; repeal ObamaCare, ditto. More eye-catching are his boyish good looks, a gift for speechmaking, and inspirational biography and ethnic roots. Yet those were Barack Obama’s attributes as well. Two years into his presidency they are devalued novelties, no matter which party card you hold.

Something else accounts for Mr. Rubio’s rise from a blip in polls against the popular governor in his party, to the runaway favorite tomorrow. He appealed as a different sort of Republican. He kept his pitch upbeat, shunned personal attacks, worked hard to widen support without apologizing for his conservatism, and more noticeably than anyone in this race ran on an unabashed and constantly invoked faith in American exceptionalism.

Bill Clinton sure noticed his success, and recently the former president threw a Hail Mary to stop him by nudging Kendrick Meek—the Democratic nominee who won a contested primary—to leave the race. The deal was intended to get Democrats behind Charlie Crist, who in April fled the GOP in the wake of the Rubio stampede to run as an independent and now stands in second place. Mr. Meek refused, leading the Clinton and Crist camps to get the story out last Thursday night to sap his support. Mr. Crist kept up the charge, calling Mr. Rubio “a tea party extremist” and “right-wing radical.” Throughout this race, Mr. Rubio has inspired venom.

His response stayed in campaign character. He didn’t call out any of the antagonists by name and repeated everywhere that “this story” showed what’s wrong with insider Washington politics. His rallies are largely free of common GOP swipes at Obama, Pelosi and Reid. It’s mostly earnest talk of governing philosophies and America’s virtues.

A small example: Driving back from a television interview here Friday morning, I bring up one media shorthand for him, “the Republican Obama.” He cracks, “Well, I don’t even think Democrats want to be ‘the Obama,'” then he catches himself. “I don’t have anything personal against the president,” he says. “It looks like he’s a really good father, looks like a really good husband, which are his most important jobs.”

The GOP’s cranky side seems to bother him. He argues that the Republican Party needs to offer up clear alternatives to liberal policy, not just say no, and brighten its tone along the way. Take immigration. “Where Republicans have failed: We should be the pro-legal immigration party, not the anti- illegal immigration party,” he says. If he wins, Mr. Rubio will be the most prominent elected Hispanic official in the U.S. from either party.

The immigrant experience provides the raw material for his most resonant message. Mr. Rubio’s parents fled Cuba and worked blue- collar jobs all their lives. He paid his way through college and law school.

“The only privilege that I was born with was to be a citizen of the greatest nation in human history,” he tells a breakfast crowd of supporters at the Original Pancake House in Palm Beach Gardens. “What makes America great is that anyone from anywhere can accomplish anything.” The Obama agenda puts this unique inheritance in jeopardy, he says. Yet he keeps it all upbeat, inclusive, and to many people who see him in person, Reagan-esque.

“He’s the only guy I know on the scene today who makes grown men cry,” says Jeb Bush. Mr. Rubio is a political protégé of the former Republican governor. They share a preference for (in Mr. Bush’s words) “hopeful aspirationalism” to broaden the party’s appeal. He adds, laughing, that the younger man is a “much better speaker than I am.”

Coming from a Bush, the joke brings home another truth. The GOP hasn’t been blessed with good communicators the last two decades. Mr. Rubio is articulate, highly disciplined and gaffe-free. With audiences he projects a warmth harder to feel in person, where he can come off cool, somewhat aloof, unless you get him talking about his love for football. At 39, the Gen-Xer represents a new generation of Republican leaders.

Democrats who knew him in his days as speaker of the Florida House of Representatives in 2007-09 shake their heads at the transformation. At first, the local press didn’t take his candidacy seriously. The campaign went around them and the party establishment. The Web and the tea parties helped. The national conservative press did as well by bringing him the sort of attention he was denied in Florida.

It was a long shot. His strategist Todd Harris remembers a low point in the second quarter of last year, when Mr. Crist raised $4.3 million, Mr. Rubio just $340,000. When his GOP primary run took off and prompted Mr. Crist to try his luck as an independent, the governor led the polls, topping out during the Gulf oil spill.

But Mr. Rubio took and widened the lead from August on. He softened his views on Social Security reform to keep Florida seniors on board. He won over some other surprising converts. Six major Florida papers that tend to prefer Democrats, including the Miami Herald, endorsed him.

The years in Florida politics leave some blemishes. The Crist campaign pushed the story that Mr. Rubio misused a party credit card, among other ethical lapses. None of it stuck, to the frustration of Democratic strategists, who insist that in any other year the allegations would have sunk him. Rubio advisers say at worst they amount to sloppy accounting on his part. But these charges will follow him beyond Tuesday, assuming his own aspirations are larger.

Mr. Rubio waves off the national attention as “fleeting most of the time.” “Politics is full of one-hit wonders,” he says. He adds that his focus is only on tomorrow. But his lead—at 19 points in the Real Clear Politics average of the polls—looks insurmountable. It might be noted that the race for governor, by contrast, is a toss-up.

Who knows what sort of senator Mr. Rubio might make, or how politics will play out in coming years. He has never met or spoken with Sarah Palin, who backed him. But with the political persona unveiled in this campaign, it’s not hard to imagine him taking her place as the Republican in America who turns the most heads. For many people to his left, that’s a scary prospect.

Mr. Kaminski is a member of the Journal’s editorial board.

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