Here's what that map would look like:
The Republican map — whether with Trump, Cruz or the ideal Republican nominee (Paul Ryan?) as the standard-bearer — is decidedly less friendly. There are 13 states that have gone for the GOP presidential nominee in each of the last six elections. But they only total 102 electorate votes. That means the eventual nominee has to find, at least, 168 more electoral votes to get to 270. Which is a hell of a lot harder than finding 28 electoral votes.
Many Republicans — particularly in Washington — are already preparing to blame a loss this fall, which many of them view as inevitable, on the divisiveness of Trump. That's not entirely fair to Trump, though.
While his dismal numbers among women and Hispanics, to name two groups, don't help matters and could — in a worst-case scenario for Republicans — put states such as Arizona and even Utah in play for Democrats, the map problems that face the GOP have very, very little to do with Trump or even Cruz.
Instead they are, largely, demographic problems centered on the GOP's inability to win any large swath of nonwhite voters. New Mexico, a state in which almost half the population is Latino, is the ur-example here. In 2004, George W. Bush won the Land of Enchantment in his bid for a second term. (His margin over John Kerry was 588 votes.) Eight years later, Barack Obama won the state by 10 points over Mitt Romney; neither side targeted it in any meaningful way.
What has become increasingly clear is that any state with a large or growing nonwhite population has become more and more difficult for Republicans to win. Virginia and North Carolina, long Republican strongholds, have moved closer and closer to Democrats of late. (Obama won both states in 2008 and carried Virginia in 2012.)
What Donald Trump is doing on the campaign trail
At the same time as these states have grown friendlier to Democrats, there are very few states that are growing increasingly Republican. Wisconsin and Minnesota are two, but neither is moving rapidly in Republicans' favor just yet.
What you are left with then is an electoral map in which the Democratic nominee begins at a significant advantage over the Republican one. (It is the obverse of the massive Republican electoral college edge of the 1980s.) And that edge is totally distinct from any individual candidate and his/her strengths or weaknesses. Yes, Trump as the nominee is more problematic than Ryan as the nominee, but the idea that Ryan would start the general election with a coin-flip chance of being elected president is just wrong.
The Republican map problem goes deeper than Trump — or any one candidate. Blaming Trump for a loss this November not only misses the point but could ensure that Republicans are doomed to repeat history in 2020.