If only one poll showed Trump losing to Clinton, perhaps you could dismiss as a bad poll. But when 50 out of 56 polls show Trump losing, often in spectacular fashion, you can't dismiss the results as inaccurate. Trump would be a disaster for the GOP - perhaps costing them the House and definitely the Senate. The reason Trump is behind and the reason he will lose is simple - only 35% of voters have a favorable opinion of him. Fully 63% do not like him - that's a deficit of 28 percentage points. Clinton is unpopular as well, but her unfavorable number is 55% and her favorable is 41% - so her deficit is 14 percentage points, exactly half the size of Trump's.
Folks need to remember that primary voters and general election voters are very different. So far, Trump has carried about 40% in the primaries, primaries in which about 17% of Republicans have voted. That translates to 7% of all Republicans. Only about 30% of registered voters are in the GOP. They key to winning is winning over non-GOP voters. And Clinton's unavailability is driven large by exceptionally high unfavorable ratings from registered Republicans - voters she was never going to win. Trump, however, has high unfavorable numbers from Democrats as well as Independent AND Republican voters.
Republicans seem to think that a majority of voters dislike Clinton as much as they do. They think there is no way voters would choose Clinton. This belief is driven by the echo chamber effect - the anti-Clinton folks talk with other anti-Clinton folks so they only hear about disapproval. They also think everyone hates Obama, but Obama's approval rating has rebounded and is back over 50% - it's actually 53%. One clear indicator of whether the party in power will win an election is the approval rating of the president - if it's above 50% the party in the White House usually wins.
If Trump is the nominee, I think a lot of Republicans are going to be completely dumbfounded on election night as they watch Clinton win, Democrats retake the Senate, and possibly the House. At this point, the GOP's best hope for defeating Clinton is a contested convention that picks a nominee other the Trump (or Cruz who loses to Clinton in 44 out of 56 polls and has an unavailability deficit of 20 percentage points.)
So who beats Clinton? John Kasich. He leads in 11 out of 16 polls and in 10 of 10 of the most recent polls. What's Kasich's favorability deficit? He doesn't have one. He actually has a favorability surplus of 7 percentage points - interestingly, his average lead over Clinton is just about 7%. Kasich may not winning among the 7% of Republicans who have voted in primaries, but he's clearly the preferred choice among those folks who will vote on Election Day.