There will be a bloodletting in the Democratic Party
For many years, there has been a marked divergence in behavior between Republican and Democratic base voters. Broadly speaking, liberals want compromise, and conservatives don't. Because the extremely poorly-designed American Constitution is constructed to require compromise, this gave Republicans a large advantage during times of divided government.
With President Trump, that is changing, and fast. He came into office losing the popular vote by the biggest margin ever, with a substantial assist from Russian spies and an even bigger one from the FBI, and now Republicans are seizing the chance to jam through a huge raft of horribly unpopular legislation. Ordinary rank-and-file Democrats are seething with fury, and demanding no compromise with Trump.
Anybody who's up for re-election is going to have to channel this energy.
Senate Democrats are, most of them, extremely confused by this sentiment. They are the first target of liberal outrage, since they have to vote on Trump's Cabinet nominees. They don't control the chamber, so it mostly doesn't matter in substantive terms how they vote — but it's still a powerful symbolic act. (Though they could have come close to picking off the wretched Mike Pompeo as CIA director, since Rand Paul voted against him.)
But not a single Democratic senator has voted against every nominee, as Paul Blest points out. Only Kirsten Gillibrand and Tom Udall have come close, voting against five of six. Six other caucus members have voted against four of six: Bernie Sanders, Cory Booker, Richard Blumenthal, Jeff Merkley, Ron Wyden, and Elizabeth Warren. On the other hand, fully 14 Democrats have voted for all six of Trump's nominees — and some of those in safe blue states, like Dianne Feinstein (California), Brian Schatz (Hawaii), Sheldon Whitehouse (Rhode Island), and Chuck Schumer (New York) — who is also the minority leader.
I can sort of see the idea behind voting to confirm somebody like Gen. James Mattis as secretary of defense, who is against torture and seems like he might be a bulwark against Trump's erratic personality. But again, he was going through anyway. At that point you might as well go for symbolism and rallying the base, since there is literally no downside. And there is no conceivable justification for voting for the torture-curious Pompeo for any position of any kind, anywhere.
Democrats are stuck in an antiquated, genteel model of how the Senate is supposed to operate. The president needs to staff his Cabinet, and so back in the day, unless somebody was really terrible, the norm was that he should basically get to pick who he wants. And indeed, when Obama first took office, he got reasonable deference — though Senate Republicans did block Judd Gregg, and Tom Daschle had to withdraw due to ethics violations not one-hundredth the size of those of Treasury nominee Steven Mnuchin.
But for the most part, Republicans mounted total procedural obstruction to Democrats and President Obama, and it only worsened as his presidency passed. The goal, as then-Senate Minority Leader Mitch McConnell said in 2010, was to "deny President Obama a second term." They filibustered nearly every bill, even ones that would go through 100-0, simply to gum up the calendar and eat up precious floor time. They filibustered nearly every judicial nominee (until Senate Democrats scaled back the filibuster), to keep liberals out of the courts — and last year, when Antonin Scalia died, Senate Republicans refused to even consider Obama's Supreme Court nominee for an entire year, in hopes that Trump would be able to fill the seat. That has literally never happened before.
This has been a nihilistic, will-to-power struggle for years now, and obviously so. Republicans now control the whole government due to happenstance and the idiotic Electoral College, but they're not moderating their policies to the slightest degree out of some sense of decorum. Instead, they're going to ram through their agenda as fast as possible, and try their utmost to disenfranchise enough liberals and rig the election procedures such that America becomes a permanent one-party state.
It only takes one party to start a fight, and when you're already in one compromise is a guaranteed way to lose. Ordinary Democrats are finally seeing this truth, as shown by the gigantic marches all over the country during inauguration weekend, and later ones in Philadelphia and New York against Trump's anti-Muslim policies. Not even a week into his presidency and Trump is already facing massive unrest.
Elected Democrats are going to need to ditch their usual cringing, timid, compromising ways if they want to have a chance at a political career in the future. Even fairly milquetoast liberals are crying out for some sort of firebrand to lead a ferocious, determined resistance. If, say, Tom Udall or Kirsten Gillibrand can realize this, their national profile will quickly grow.
But those who vote for Jeff Sessions to become attorney general might face a primary challenge instead.