POLITICO: Bernie Sanders’ winning streak – which includes blowout victories over Hillary Clinton in three states on Saturday – has helped him narrow the race for the Democratic presidential nomination. But if Sanders is going to surpass Hillary Clinton in pledged delegates to this summer’s convention in Philadelphia, he’ll need to build on that momentum and win by wide margins in the types of states where Clinton has prevailed thus far – and Sanders currently trails in most of them.

A POLITICO analysis underscores Sanders’ challenge: Even after runaway victories in Idaho, Utah, Alaska, Hawaii and Washington this week – he still trails Clinton by between 200 and 250 pledged delegates, pending final tallies from this week’s races. And while Wisconsin’s April 5 primary and Wyoming’s April 9 caucuses seem well-suited for the Vermont senator, the Democrats’ proportional delegate-allocation rules limit the extent to which he can eat into Clinton’s advantage. Looking forward past the next two weeks, he currently trails Clinton in public polling in the four largest states left on the calendar: California, New Jersey, New York and Pennsylvania. With the exception of his win in Michigan earlier this month, Sanders hasn’t yet demonstrated he can win large states like these – let alone by the wide margins he’ll need to close the pledged-delegate gap.

Another problem for Sanders: He has excelled in caucus states – winning 10 of 12 (including the last 10 consecutively). But most of the states left to come are holding primaries, including a number of contests only open to registered Democrats. And Clinton has another insurance policy: the more than 700 so-called “superdelegates” who can support the candidate of their choosing on the convention floor, regardless of the primary or caucus results in their home states. Of the roughly 500 who have publicly expressed a preference at this stage of the campaign, about 95 percent are in Clinton’s corner.

Sanders’ chances of winning the nomination outright are slight: He would have to win slightly fewer than 60 percent of the remaining pledged delegates to surpass Clinton – a huge improvement over the roughly 45 percent of pledged delegates he’s captured so far. In a best case scenario for Sanders, the results from this week represent the start of an unprecedented run that will give him momentum in the other states on the calendar and also help him peel off undecided superdelegates or even some already backing Clinton. […]

The danger for Sanders is that – while he will continue to chip away at Clinton’s delegate lead over the next two weeks – the front-runner could reassert her dominance in late April states like New York, Maryland and California. And even if she finishes short of the overall threshold just with pledged delegates, Clinton would still have the plurality and only need a small number of superdelegates to put her over the top. If Sanders is going to scramble the delegate math, it begins with Wisconsin next week. Here’s Sanders’ tight-rope path: MORE