Eight years ago, Rep. Mike Castle of Delaware was headed to the U.S. Senate. Joe Biden had left his seat open upon ascending to the vice presidency, and Castle, a Republican, appeared to be in the perfect position to take it. Delaware is a solidly blue state, but Castle had built a moderate record, was well-liked and held an early lead over his three potential Democratic opponents. His only problem: That moderate record cut both ways. By September 2010, the tea party wave was cresting and Castle got swallowed up. He lost the Republican primary to tea partier Christine O’Donnell. Two months later, O’Donnell lost the general election by 17 points, and Republicans failed to win a majority in the Senate.

The Castle-O’Donnell primary should be a cautionary tale for Democrats now. Some liberal activists want to challenge Sen. Joe Manchin in West Virginia’s 2018 Democratic primary. They complain that he’s too conservative and that he voted to confirm most of President Trump’s Cabinet officials. Manchin is probably safe — Democratic voters in West Virginia are pretty conservative.

The state tied for the highest percentage of Democratic primary voters in 2016 who called themselves moderate or conservative (55 percent). But the impulse to challenge Manchin from the left could be dangerous for Democrats. Manchin, even though he often votes with the GOP, is incredibly valuable to the Democratic Party compared to any plausible alternative.