|Damn you people with your facts and your math|
A very good example of this is the Sanders mantra 'break up the TBTF banks'. Now, virtually all liberals would agree that this is a worthwhile policy goal. But we also point out, that as federal FinReg policy, it's probably not the most effective, and should perhaps be considered as a secondary option in an array of regulatory actions. Breaking up the banks does nothing to deal with the biggest problem, so-called 'shadow banking', and provides some incentives for banks to merely spin off a number of entities that can all grow right up to the designated TBTF threshold. Most economists think hard limits on leverage - significantly greater capital requirements - and a small tax/fee on certain kinds of high speed electronic transactions would protect the economy in a more robust manner, and make for a regulatory system much more difficult for the 'masters of the universe' to game out. But if you make that case, Sanders supporters will just accuse you of being corrupt pro-bankster.
Another trend that seems to keep growing is a kind of an 'ideological purity' conservative analog within the American Political Left. That is, the rise of a significant bloc of liberal activists who reject the slow pace and hard work of everyday real-world politics and demand complete ideological purity around a set of fairly radical policies. The obvious fact that many of these high-cost/high-tax government services and strong anti-capitalist government intervention in the private sector are not only unpopular politically, but actively opposed by a larger and more powerful opposition doesn't seem to matter. They have no answer to address the political realities standing between them and their goals, but they also don't seem to feel they need one. Just as the Tea Party bloc chafes in increasing frustration in the face of their inability to overcome basic constitutional limitations like a Presidential veto, this liberal-hating liberal bloc demands the implementation of the policies THEY prefer in the face of a huge, generational Republican hold on the House of Representatives, a broadly conservative federal judiciary and widespread conservative state-level governance. Again, ask them how they would overcome those roadblocks, the answer is "the people will rise up and sweep them away".
Perhaps this presumption that there is, somewhere, hiding in the nooks and crannies of the American electorate, a huge pent up demand for far left public policies is the greatest delusion of this political bloc. They told us people, particularly young people, would rise up and sweep Bernie Sanders into the White House, but at this point he's received millions of fewer votes than the more traditional Democrat Hillary Clinton, and young people continue to vote in very small numbers - as they always have.
At the end of the day, politics is about thoughtful, incremental change - revolutions are very rare, and they require some kind of triggering factor. Wealthy, safe, comfortable societies are typically not driven to radical political change. I am personally predisposed to technocratic solutions, where the process is as important as the goal. If you choose a tremendously ambitious goal and don't have a realistic, detailed plan for achieving it, you not only will fail, but you will open up the opportunity for the opposition to exploit your failure. Public policy is made by working together with the various factions and stakeholders, compromising to get to incremental progress. A scorched earth refusal to negotiate policy solutions in good faith is a certain path to retaining the status quo.