Trump lost a series of swing counties that historically Republicans need to win the state.
These are Jefferson, Arapahoe and Larimer. Arapahoe appears mostly out of reach for Trump in 2020 but Trump absolutely has to win Jefferson and Larimer to win the state. He would need to cut the margins in Arapahoe down and get more of the vote in Denver.
This area lost him the state and it probably cost McCain and Romney the state as well.
Not just probably; it
did. The last time that Colorado voted Republican at the presidential level was when these three counties did so, and that was in 2004, when George W. Bush won these three counties (as well as Broomfield) and won the state as a whole. Now, it is possible for Republicans to win without Jefferson and Arapahoe. In 2014, Gardner lost those two counties to Mark Udall by razor-thin margins, while carrying Larimer by a razor-thin margin. He was able to defeat Udall because of running up the score in El Paso, Douglas, Mesa, and Weld (the Republican base counties), and by demolishing Udall in the Plains and Mountain counties.
Trump has no chance of winning Colorado next year, but if he did, his path to victory would resemble Gardner's, though he would probably get slightly lower percentages in El Paso and Douglas (which are trending Democratic, if only marginally), and would sweep the block of historically Democratic counties in Southern Colorado-Pueblo, Huerfano, Las Animas, and Conejos, which continued their Republican trend last year. Polis lost the latter two counties, and only one statewide Democrat (Jena Griswold) carried all four. Conversely, however, Stapleton lost Jefferson, Arapahoe, and Larimer by double digits, and of course, lost Broomfield by double digits as well. I wouldn't be surprised if Trump suffered a similar fate.
