Why does the Democratic Party hate Joe Sestak so much?

He's in line with the Democratic Party ideologically. He served as a Democrat in Congress for two terms. He's a former Navy admiral (three stars!) and a former Clinton White House staffer. And when he ran for the Senate in Pennsylvania in 2010 — an objectively terrible year for Democrats — he lost to Pat Toomey by only 2 points.

But when he decided to run against Toomey again in 2016 — launching his campaign by crossing the state of Pennsylvania on foot — the Democratic Party spent months desperately trying to find someone to run against him. And when their top picks demurred (or got indicted), and they ended up with Katie McGinty — a former chief of staff for the state's current governor, who'd placed last in the Democratic gubernatorial primary in 2014 — they pulled out all the stops for her.

Everyone from Barack Obama and Joe Biden on down has endorsed McGinty. Former Pennsylvania Gov. Ed Rendell is running her campaign. The Democratic Senatorial Campaign Committee has sent over $1.5 million McGinty's way; no other non-incumbent Democrat has gotten more than $14,000. In total, outside groups have spent $4 million on her.

This isn't about love for Katie McGinty. It's about distrust of Joe Sestak. But what could have earned such skepticism from party elders?

The party's beef with Sestak goes back to his first Senate run in 2010, when he successfully defeated Republican-turned-Democrat Arlen Specter for the Democratic nomination.

In a cycle when Democrats might retake the Senate — with Pennsylvania as one of their most winnable seats — the resistance to Sestak is raising questions about at what point personal stubbornness becomes a campaign liability, for candidates and parties alike.

Sestak embarrassed Democrats in 2010 by running against an ex-Republican

In April 2009, Pennsylvania Sen. Arlen Specter left the Republican Party he'd been a member of for 40 years and declared himself a Democrat.

It was a huge victory for the Democratic Party. Politically, it helped them claim that they were the party of reasonableness while Republicans were the Tea Party-crazed "party of no." From a legislative perspective it was even more important: It ensured that (once Sen. Al Franken was formally seated) Democrats had a filibuster-proof 60-vote majority — which they used to pass the chief legislative accomplishments of the Obama administration, the Affordable Care Act, and financial reform, in 2010.

And in return, Democrats, including President Obama and Gov. Rendell, promised Specter one little thing: that they'd support him when he came up for reelection in 2010, in both the general election and the primary. Compared to the Republican primary, where Specter was facing a rematch with Pat Toomey — in a cycle where Republican voters were getting extremely restless about incumbents — the assured backing of the Democratic Party and the president of the United States seemed like a definite improvement of job security.

But Joe Sestak had been considering a Senate run, and he hadn't made any promises to anybody. Sestak sought the advice of ex-boss Bill Clinton about what to do; Clinton reportedly told Sestak, "Go with your gut." So Sestak did just that, and jumped into the race.

Other national Democrats eventually forced Clinton to get on the phone with Sestak and ask him to drop out, but by that time it was too late.

Sestak's deep-seated distrust of the establishment (earlier this year, he managed to stretch a 10-minute interview about it into a 90-minute complaint session) would not abate even after he won the Democratic nomination from Specter. He refused to participate in the statewide Democratic "coordinated campaign" for fundraising, and, in 2012, he supported some primary candidates challenging incumbent Democrats in Congress.

All of this might have been forgiven if Sestak had at least secured the seat for Democrats in November 2010. But he didn't.

"He's much better for them than Toomey, on his most mavericky day"

There's no denying that Sestak's decision to run in 2010 made a lot of Pennsylvania's leading Democrats (not to mention the president) look very bad. "If they couldn't get [Specter] the Democratic nomination," points out University of Maryland political scientist David Karol, "that doesn't encourage anyone else to do what he did."

On the other hand, Sestak was simply doing something that plenty of Republicans — and a few Democrats — have done over the last several cycles: run against an incumbent that was far more centrist than the party's base, in the hopes of getting elected as a candidate who represented the party.

After all, as much of a pain in the butt as Sestak is to the institutional Democratic Party, he's ideologically in line with them. "When I was in the House, I didn’t usually let them know until the last day [how I was going to vote]," he told a Politico reporter in April. "But then again, look at my voting record. Oh, my gosh! 100 percent voting record with AFL-CIO!"

The bottom line, says Karol, is this: "he's much better for them than Toomey, on his most mavericky day."

The Republican Party has often failed to protect incumbents facing primaries over their ideological purity; in the 2012 cycle, the official National Republican Senatorial Committee didn't even protect any incumbents. Would the Democrats really punish someone who tried that challenge not once, but twice?

When is stubbornness an electoral liability?

Anti-Sestak Democrats (which is to say, most of them) argue that whatever kind of senator Sestak might be, he wouldn't be a very good candidate. He lost to Toomey once, and they don't feel he's learned enough not to do it again.

What's odd about this claim is that Sestak appears to be a relatively strong candidate for the state — and, generally, parties are willing to put up with a little maverickiness in the service of electability. Sestak may have lost in 2010, but he ran 7 percentage points ahead of the Democratic gubernatorial candidate. And this time around, at least as of mid-March, Sestak was a few points behind Toomey in polls — but McGinty was way behind Toomey.

Furthermore, it's entirely possible that Sestak will beat McGinty. Even with the high-profile endorsements and insider track, it took McGinty until last week to pull even with Sestak in (some) primary polls.

Read between the lines of establishment Democrats' complaint, and it begins to seem that Sestak's lack of "electability" is rooted in the fact that he doesn't listen to national Democratic strategists.

At the beginning of the 2016 cycle, the Democratic Senatorial Campaign Committee suggested a potential campaign manager for Sestak (which is fairly typical for candidates accepting campaign-committee money, especially when they don't have much experience with statewide elections). But Sestak claims — in an anecdote he's turned into his stump speech — that a DSCC official told him that he had to say "yes" to everything he was told to do.

To Sestak, his refusal to be a "yes" man is evidence of his independent spirit (which is why it's a stump-speech anecdote). To the party, it's evidence that Sestak doesn't actually know how to run for office. Mere weeks before the primary, Politico said that "DSCC officials don't even know who's running his campaign." (It's a group of local Pennsylvania operatives, but one assumes the point of the DSCC's snub was that they weren't anyone worth knowing.)

Or take the decision to walk across the state to launch his 2016 campaign. National Democrats didn't get it — they saw it as a pointless stunt. To Sestak, that was just evidence that they were out of touch themselves.

It's not unreasonable to define "electability" as "skill in running an electoral campaign." McGinty might not have that, but she's proven she's willing to listen to the people who do. Meanwhile, candidates who have initially appealed to voters as "mavericks" have often lost that appeal once their refusal to stay on message causes them to say something dumb (see: Todd Akin).

But it's also not unreasonable to define it as "polling well" and "voters liking you." And on that count, it certainly doesn't look like Sestak is at a disadvantage against McGinty. To the contrary, if anything, it looks like he'd be a slightly stronger opponent for Toomey.

If Sestak wins tonight, he's said he's open to at least a "civil union" with the Democratic Party. It's not clear what his terms would be. But it's certainly going to be in the party's interest to throw its support behind the man it's spent two election cycles fighting.

"If they can't get past this if he's the nominee," says Karol, "that would really be remarkable." It would be an object lesson in when a personal grudge can get in the way of a strategic goal.