Donald Trump appears close to invincible in the early Republican primaries and caucuses, but his strength among general election voters remains unclear.
That is how basically every election has worked for 30+ years. About 40% of people lean republican, 40% democrat, and 20% independent. Campaigns are mostly about getting 100% of your 40% to vote than winning independents or opponents.
Way less people are Republican, but only in pure numbers, which doesn't mean shit when you use the Electoral College (or have 2 senators per state regardless of population, for that matter). The only voters who actually matter for presidents are in the 5 or so states that don't always go red or blue.