The difference between Bernie and most of the political establishment is that Bernie will fail trying to keep his word, whereas establishment candidates treat their promises as mere stepping stones to get where they need to go. They have no intention of keeping their promises, they just say them so people will vote for them.
Sanders himself had a near miraculous about-turn on immigration once he was no longer campaigning in an almost entirely white state while being funded by large unions which oppose more relaxed immigration (and amnesty) laws (on the basis that they drive wages down) and was instead campaigning on a national ticket where being seen as anti-immigrant on the Democratic side of the fence is seen as a death sentence.
I don't want to go over the top in my criticism of Sanders on that; he generally has stuck to his principles. But he's a career politician and part of being a career politician is doing the somewhat unseemly part of politics. And that includes changing your positions when political advantageous.
In my ideal world, politicians would wake up tomorrow and start doing their jobs to serve this country rather than serve themselves; they would stop taking money from corporate donors to aid in their re-election campaigns and rely on building the goodwill of the people back home (through doing their jobs) in order to achieve those results; they would recognize that Congress is not a place you're supposed to spend your entire working life; they would stop being politicians and start being statesmen again. But I don't believe that's going to happen, because we cannot expect a bunch of egocentrists to think about anyone other than themselves.
You know this basically sums up Trump right?
While there's no doubt that at least part (and I imagine a pretty significant part) of his Presidential run is to massage his rather large ego, Trump would have a far easier, happier and likely monetarily more successful life if he hadn't run; he's doing anything but serving himself. Hell, he's basically the only candidate who will be materially worse even if he does
win due to the amount of his own money he's put into his campaign; other candidates are getting to play with other people's money.
Linked to the above, virtually no money from corporate donors (far less than Sanders has received from the unions and vastly under what other candidates have got from corporations)
Hasn't spent his entire working life in congress (or even politics... unlike Sanders)
He may not be statesmanlike
but he's taken the lead on issues such as immigration and China's aggressive economic policy. Hell, he stood up in the middle of the Republican
debate, said the Iraq War was a mistake, called out George Bush and is still
the front runner.
There's also the nebulous definition of "doing your job". Take pork barrel spending for example... "bridges to nowhere" and the like. Almost no-one likes it, it seems corrupt and ridiculous and a prime example of, depending on your take on such things, the government being far too large or spending it on the wrong projects. Yet if you're a Representative and you can get the Federal Government to invest $300 million in your state next year, providing jobs and income to thousands of people you represent, isn't "doing your job" (i.e. trying to make life better for your constituents) getting
that money, even if it is being spent on a bridge that no-one will use? Around half a billion dollars have been spent over the past couple of years on upgrades to the Abrams tank that the army don't want and would prefer to see spent somewhere else... seemingly a perfect example of politicians not doing their job well. But if you're a politician in Ohio, home to the Lima Army Tank Plant (the sole producer of Abrams) and you know that such spending guarantees jobs, wages and investment in Ohio which will benefit the people you represent, isn't it your job to fight for
that money... and to get even more if possible?
I'm also not sure how a wish for politicians to "do their job" interacts with your other stated point that it was a problem that Obama negotiated and compromised in the name of getting things done, rather than making a stand and refusing to budge at all; isn't part of doing your job as a politican in a system like the US's compromising and negotiating to get things done rather than finding a hill to die on?
On the wider point about how Sanders will deal with the Republicans and trying to "break" them, let's remember this isn't a one way street. The Republicans have a decent chance of retaining both House and Senate in the coming elections which mean they're
the ones in the driving seat when it comes to putting legislation through. Yes, they may use that power to block any of Sanders suggestions that he gets the minority Democrats to propose... but they'll likely also use it to put through their own legislation, forcing either the Democrats to stall and filibuster or Sanders to veto. One can hardly argue that it's the other side ruining politics when you're also
stalling and then vetoing any legislation they put through.