Thomas Oliphant, center, moderates a forum for Virginia’s 5th Congressional District race between Republican Denver Riggleman, left, and Democrat Leslie Cockburn in Washington, Va., in September. (Dayna Smith for the Washington Post)

The one-time sleeper race in Virginia’s 5th Congressional District between Republican Denver Riggleman and Democrat Leslie Cockburn is now a toss-up.

Back in late May, I wrote that after incumbent GOP Rep. Tom Garrett’s decision to drop his reelection bid, Republican chances of holding the 5th District were better and would get even more so if they selected a challenger able to run a tough, aggressive, well-funded campaign.

Instead, after a bizarre nominating convention, Republicans opted for the tyro Riggleman. His messaging may fit the GOP-leaning District — it’s not wildly different from Garrett’s. But Riggleman’s fundraising has been anemic.

Those numbers, combined with the general political climate and a very well funded Cockburn campaign, have — apparently — changed a lean-GOP race into a toss-up.

Writing in Sabato’s Crystal Ball, managing editor Kyle Kondik says the 5th “is not a district that one might think would elect a Democrat under normal circumstances.”

Donald Trump won here by 11 percentage points in 2016. Ed Gillespie won the 5th by 9 percentage points in 2017.

But until the New York Times/Siena College poll was completed earlier this week, we had no independent data on the race. What we got was a statistical tie: Cockburn leading Riggleman 46-45 percent, with 10 percent undecided.

Granted, this is just a snapshot of the race. Those numbers will change. But the Times ran the data through several turnout models to see where voters may be heading.

Only one — in which the 2018 electorate looks like it did in 2014, when then-incumbent Rep Robert Hurt easily won reelection — favored Riggleman.

Interestingly, it was also the model showing the lowest turnout.

But models and turnout aren’t the bigger issues. It’s the candidates.

Kondik writes, “National Democrats and Republicans grumble about their respective candidates in the race,” with Israel-foe Cockburn possibly “too liberal for the district,” and Bigfoot fan Riggleman a little “too eccentric.”

It that seems a bit familiar, it should. In 2008, Democrat-turned-independent-turned-Republican Virgil Goode faced progressive first-time candidate Tom Perriello.

In what was the closest House race in the nation that year, Perriello, the candidate who was too liberal, was the victor.

Yes, it was a presidential election year. And not just any presidential election year, but one in which Barack Obama broke the GOP’s decades-long presidential winning streak in Virginia.

Republicans wrote Perriello’s win off as an “anomaly,” and, according to The Post’s reporting, predicted a Republican candidate would win the 5th in 2010.

That’s exactly what happened.

Does this mean Cockburn, following the Perriello example, could win — very narrowly — but flame out after a single term?

Maybe. But several pieces that helped Perriello to victory in 2008, aside from a historic presidential race, are missing this year.

Key among them: a scandal-plagued incumbent. Goode was under fire for taking illegal campaign contributions, erratic behavior and a bizarre incident involving a Goode staffer, who had a bit part in the erotic film “Eden’s Curve,” which “depict[ed] homosexual relationships that was shot in Danville.”

Not something a “staunch opponent of gay rights” could explain away.

But both of the current candidates are trying to forge links they hope the other won’t be able to avoid.

For her part, Cockburn has tried to tie Riggleman to the current, scandal-plagued incumbent, Tom Garrett, and the “far right” House Freedom Caucus.

Riggleman is touting Vice President Pence’s endorsement of his campaign and linking Cockburn to House Minority Leader Nancy Pelosi (D-Calif.).

It’s standard, if thin, campaign gruel. Don’t expect it to get any beefier between now and Nov. 6.

But do expect a surprise when the votes are counted. The winner Nov. 6 will be a political newcomer who isn’t a favorite of party bigwigs.

That’s an opportunity for the winner to immediately carve out a place and establish a platform that a cookie-cutter candidate would never be able to achieve.