From left, Sen. Marco Rubio (Fla.), Donald Trump and Sen. Ted Cruz (Tex.) were among the candidates who participated in a Republican primary debate in March. (Joe Raedle/Getty Images)

Long before the first polling places opened on Election Day 2016, the race for the 2020 Republican presidential nomination was already underway.

It has been unfolding in early primary states, where potential candidates have been introducing or reintroducing themselves. It has been on display in purple battlegrounds where they are helping in down-ballot contests. And behind the scenes, would-be contenders have sought face time with party power brokers eager to size them up.

“I think it’s already happening now,” said Rep. Tom Cole (Okla.), a longtime GOP strategist. Cole identified another significant way the prospective candidates are laying a foundation: by embracing or shunning Donald Trump.

“You’ve seen some pretty dramatic reversals of people deciding they couldn’t be for Trump and they are for Trump. Now, part of that, to me, is about positioning for presidential contests.”

South Carolina Gov. Nikki Haley and Sen. Marco Rubio (R-Fla.) have been mentioned as possible 2020 presidential candidates in the event of a Donald Trump loss. (Chris Keane/Reuters)

If Trump loses the presidency on Tuesday, Republicans will be forced to choose yet again from a full slate of ambitious candidates-in-waiting with wildly divergent visions for the party’s future. Even if Trump wins, he will begin his term far from safe against the threat of a primary challenge in 2020, because an ample cross-section of his party has spoken out against him.

A Trump win also probably would set off a potentially chaotic scramble on the Democratic side to field a challenger in four years. Democrats have not built a robust bench during President Obama’s time in the White House, in part because of the down-ballot drubbings the party has experienced in the midterm elections and partly because Hillary Clinton effectively froze the field as she considered her 2016 run.

No Republicans have announced that they will run in 2020 if Trump loses. Most have avoided even broaching the subject publicly, to avoid appearing presumptuous.

But interviews with more than a dozen Republican strategists, elected officials, donors and rank-and-file voters show that the party has begun to ponder its future options and that the auditions have started.

Among the names mentioned most often are several unsuccessful 2016 candidates who eventually — and awkwardly — came around to supporting Trump: Sen. Ted Cruz (Tex.), Wisconsin Gov. Scott Walker and Sen. Marco Rubio (Fla.).

Some consider Ohio Gov. John Kasich, who has held firm in his opposition to Trump after losing to him, a possibility. Republicans also are intrigued by Sen. Ben Sasse (Neb.), an early Trump critic.

Sen. Tom Cotton (R-Ark.) is often mentioned as a potential 2020 presidential contender. (Nikki Kahn/The Washington Post)

Trump’s running mate, Indiana Gov. Mike Pence, also comes up frequently as a possibility if the 2016 ticket loses, as does House Speaker Paul D. Ryan (Wis.), who has had a rocky relationship with Trump.

Republicans also are interested in fresher faces, including first-term Sens. Tom Cotton (Ark.), Cory Gardner (Colo.) and Joni Ernst (Iowa); and South Carolina Gov. Nikki Haley and Texas Gov. Greg Abbott.

Bobbie Kilberg, a Northern Virginia-based Republican fundraiser, said that she has been approached by possible candidates seeking meetings after the election to lay out their visions for the party’s future. She declined to say who has sought her out.

“If Trump loses, I think it is very important that we do not again have eight to 10 highly qualified center-right candidates running,” Kilberg said before Election Day, referring to the packed 2016 field. “We have to over time — not now, it’s too early — coalesce around one or two, maybe three, people on the center-right.”

However, that lane appears to be no less clogged than it was four years ago. Kasich made a late-summer return to New Hampshire, an early nominating state no politician visits by accident. Rubio was up for reelection, and his win Tuesday will help wash away the stain of a stinging home-state primary loss to Trump and give him new life in the party that some backers hope he will parlay into another White House run.

Cotton, a national security hawk who is an Army veteran of the Iraq and Afghanistan wars, spent time in Iowa last month. In the closing days of this campaign, he has hit the trail on behalf of GOP Senate contenders in tight races in Indiana, Pennsylvania and Wisconsin, a swing that could earn him some key allies down the road.

Trump’s unlikely primary triumph, which came without early support from party elites, has emboldened many Republicans to push for an outsider again in 2020.

“If there’s one thing that this campaign cycle has shown, there is some competitive advantage for folks who are not viewed as being rooted in the establishment and the status quo,” said Ken Blackwell, a former Ohio secretary of state.

Cruz, who has clashed with party leaders, claimed an outsider mantle as he ran for president this cycle. But it fell flat against Trump, and his pitch could face new obstacles if he runs in 2020. Cruz only recently backed the GOP nominee, months after declining to do so during a Republican National Convention speech that drew boos from many Trump supporters. Cruz’s most immediate priority is winning reelection in 2018.

Ryan also has a near-term campaign that does not involve the presidency: winning reelection as speaker. His bid could provide a snapshot of his standing in the party after months of a turbulent relationship with Trump. It’s also not clear that Ryan, who was the Republican vice-presidential nominee in 2012, wants to jump back into a long national campaign.

One person whose name comes up among Republicans from different ends of the ideological spectrum is Pence, who has been a loyal and on-message running mate for Trump. Pence is well liked by GOP leaders and Trump, too, making him something of a hybrid.

But whether Pence could energize Trump’s base of loyal supporters remains to be seen. His style — soft-spoken and folksy — contrasts sharply with Trump’s brashness and willingness to pick fights with members of his party, which drove his popularity in the primary.

“I’ve known a little bit about Mr. Pence. Now, Mr. Pence — I like what he has done in the state of Indiana economically overall, but Mr. Pence, he is a standard-issue Republican. I’m not a Republican. I’m a conservative independent,” said Conrad Baker, 40, who lives in St. Francis, Minn., and attended a recent Trump event in Eau Claire, Wis.

Many Republicans say they don’t expect Trump to step away from politics completely if he loses, nor do they expect his supporters to walk away from politics en masse. So the trajectory of the next primary could rest heavily on how popular or unpopular he is in the lead-up to 2020.

But Trump’s anti-immigrant, anti-Muslim rhetoric has many Republicans worried that his views are hurting the GOP’s already struggling standing among minorities and putting the party further out of step with a swiftly diversifying country.

Some Republicans said a Trump loss could spur the party to walk down a path similar to one Democrats traversed after a third straight White House loss in 1988. In 1992, they nominated Bill Clinton, a centrist who won the election, helping the party reassert itself on the national stage and shedding the image that it was too liberal for mainstream Americans. That may be easier in theory than in practice for Republicans to replicate in their next primary.

“If you look at the Republican primary electorate compared to the Democratic primary electorate in 1992, the Republican primary electorate is more homogeneous and . . . much more ideologically skewed,” said Alan Abramowitz, a political scientist at Emory University.

After Trump’s unlikely rise, which few saw coming at this point for years ago, Republicans are bracing themselves for unexpected twists and turns in the coming months and years more than ever before.

“My one prediction about 2020 is it won’t look anything like what we think it will look like the day after the election,” said David McIntosh, the president of the Club for Growth, an anti-tax group.

Jenna Johnson in Eau Claire, Wis., contributed to this report.