Whether more Dems should jump in and whether she's unbeatable are separate questions. I don't think she's unbeatable but I also am totally unsurprised that her only real challenger is Bernie, who is not even a Democrat.
To win a primary, you need support from the party establishment. Hillary's cornered that today, and she didn't do that in 2008. Even before winning Iowa, Obama was endorsed by Ted Kennedy and at least a handful of other Senators. Hillary has the endorsement of 35 of 46 D Senators and I don't think anyone else has even 1. She's had 7 years to firm up intra-party support in a way she didn't for 2008. And she's doing that by inviting people to work with her and incorporating at least some of the concerns of all the factions of the party. Her speech about quarterly capitalism yesterday is a good example. CEOs don't really love the constant short-termism and pressure, and lefties like Stiglitz and his crew at the Roosevelt Institute absolutely hate it because it's devastating for workers. And here's she's out with super wonky detail that unites economic progressives and wealthier social liberals. HRC might not be an inspiring politician but she's good at building coalitions throughout Democratic-aligned interest groups. There's no real base in the party for anyone besides a committed social democrat to scoop up. So we're in a situation where she all her opponents within the Democratic Party (i.e. not counting Bernie) are hopeless dead-enders with no electoral future.
