According to Real Clear Politics, head-to-head Trump v. Clinton polls have been done in 2016 in 20
The other states which Obama won but Trump outperforms Clinton are Connecticut, Iowa, Michigan, New Jersey and New York. Trump still though polls behind in all those states by margins of 9.5%, 5%, 8.6% and 21.2%. He's unlikely to win any of them in a general election contest.
So while the polling suggests Trump might win Minnesota's 10 electoral votes, the analysis shows his unpopularity is more likely to put formerly Republican states into play. Astonishingly Trump trails Clinton in a Utah head-to-head poll by 2%, a state Romney won in 2012 with 48.04% of the vote. But lest one think that was an aberration due to Utah's Mormon leanings, Trump only leads Clinton by 2% in Texas, a state Romney won by 15.79% Then you have North Carolina, a state that Romney narrowly won by 2.04% in 2012. In head-to-head polling, Trump loses the state to Clinton by 2%. And let's not forget Arizona a state Romney won by 9.06% in 2012. Trump loses it to Clinton by 3.5%. Wait another one. Trump only leads Clinton by 3% in Mississippi, a state Romney won by 11.5% of the vote.
(Note: Although Indiana hasn't had Trump v. Clinton head-to-head polling done, you can bet the Hoosier state will be competitive for the Democrats if Trump is the Republican presidential candidate.)
Further, a Trump candidacy is likely to make several toss-up or leaning Democratic states that Romney lost in 2012, even more Democratic. Examples include Florida, Ohio, New Hampshire, Pennsylvania, Virginia and Wisconsin.
Below is a table of the states where Trump underperforms Romney's 2012 numbers.
State T v. C Poll 2012 Winner 2012 Margin
The effect of Trump turning red states blue could have a devastating effect on the GOP's efforts to hold onto the House and Senate. While a Ted Cruz might not win any more electoral votes than Mitt Romney did, you can bet his candidacy would at least help the Republicans hold onto states they had in 2012 and give the GOP a fighting shot to hold onto both the House and Senate.