Democrats and Republicans don’t agree on much these days. But they do agree on one thing: Rep. Darrell Issa, R-Calif., is in danger.

Since 2001, Issa has represented the coastal suburbs of northern San Diego and southern Orange counties. Reliably conservative, California’s 49th congressional district has never really been much of an electoral battleground. Issa won his first House election by 33 percentage points, and for the next 16 years, no challenger ever came within 15 points of unseating him.

Until 2016, that is.

Last November, Issa squared off against the most serious opponent of his career: retired Marine Col. Doug Applegate. To everyone’s surprise, the Democratic rookie nearly won, finishing a mere 1,621 votes — 0.6 percentage points — behind his entrenched Republican rival. Meanwhile, California as a whole voted for Hillary Clinton by nearly 30 percentage points, and Issa’s district went for Clinton, too (by nearly 9 points).

Today, only 23 other GOP members represent districts that Clinton won; of them, Issa was reelected by the slimmest margin. This makes him, as the New York Times recently put it, “the nation’s most vulnerable incumbent.”

Issa already positioning himself for a close contest in 2018. Famous for what Mother Jones has called “his high-theater, low-yield investigations into alleged Democratic scandals involving Benghazi, the IRS, the gun sting gone awry known as Operation Fast and Furious, and Healthcare.gov, among others,” Issa has been trying to project a more bipartisan air since his near-death experience last November. He proposed one of the GOP’s first concrete Obamacare replacement plans. He backed the idea of an independent investigation into President Trump’s ties to Russia days before Attorney Gen. Jeff Sessions recused himself from the case. Last week, despite a long history of climate-science skepticism, he even joined the House’s Climate Solutions Caucus. And while Issa has yet to hold a formal town hall with his constituents, he did buck the trend among his fellow Republicans during last week’s recess and spent 90 impromptu minutes talking to protesters outside his Vista office.

Both the Democratic Party and the GOP have taken note. In January, the Democratic Congressional Campaign Committee named Issa as one of its top targets in 2018; a few weeks later, the National Republican Congressional Committee added him to its 10-member Patriot Program, a special fundraising and campaign operation that helps endangered incumbents keep their seats.

Even at this early stage, then, it’s clear that Issa’s re-election battle — likely to be against Applegate, who immediately announced that he wanted a rematch — is shaping up as one of the most fascinating races of 2018.

But ultimately, the most fascinating thing about it may be the fact that Issa isn’t alone. There are actually four House Republicans whose districts overlap with Orange County — and all of them rank among the top 25 most vulnerable Republicans in the country.

For any student of American political history, this should come as something of a shock. In the 1960s, Orange County was the heart of the conservative movement, fueling the campaigns of Barry Goldwater and, later, Ronald Reagan. Jerome Himmelstein, author of “To the Right: The Transformation of American Conservatism,” once called it “the most important place in the country, if you’re looking at the long-term rise of the right.”

Only four Democrats have carried Orange County in a statewide race in the last 50 years, defying California’s increasingly leftward tilt, and the county voted for the GOP candidate in every presidential election from 1936 to 2012, when Mitt Romney defeated Barack Obama locally by 6 percentage points.