Throughout the summer, the western United States plays hosts some of cycling's hardest races and events. The Leadville 100, FireCracker 50, Crusher in the Tushar, Steamboat Gravel and the Breck Epic.
These races are all decided by the rider's ability to maintain high outputs for over 30 minutes at high altitude. To put it into perspective, the high point in the Tour De France is frequently between 7,000 and 9,000 feet, whereas the Leadville 100 never goes below 10,000 feet!
Needless to say - if you're racing at elevations above 8,000-9,000 feet, you had better understand how to modify your race strategy.
The Effect of Racing at Altitude
As you gain altitude there is a reduced amount of pO2 (partial oxygen pressure) meaning that there is less oxygen for your blood to carry to your muscles. The USA Pro Challenge takes place mostly above 7,000 ft and at that altitude, the body will take in at least 25 percent less oxygen per breath because of the reduced amount of pO2 in the air when compared to sea level. With less oxygen available to deliver to the muscles, riders will see a decrease in performance when compared to lower altitudes.
The effects to your body when racing at altitude are higher heart rate and lower power output. Since you are getting less oxygen to your muscles your body increases its heart rate to help bring in more oxygen which means you reach your max output quicker. This leads to a lower Functional Threshold Power (FTP) and also makes it harder to recover from maximal efforts.
Acclimating
In preparation for these events, many racers have arrived in Utah and Colorado well in advance to help acclimate to the altitude. Since there is less oxygen being delivered to the muscles the body will produce more red blood cells which deliver the oxygen to the muscles. By spending this extra time at altitude the racers will have more red blood cells and become more acclimated which will lead to a lower drop-off rate in power at higher altitudes.
Many riders will take this approach when preparing for any race. They will have specific altitude training camps. By spending more time at higher altitudes they can increase their red blood cells, allowing them to make longer and harder sustainable efforts at lower altitudes. But they will only want to stay up for three to four weeks. Any longer than that and they will begin to lose muscle from riding at the lower power outputs. There are other methods as well such as training low sleeping high and training high sleeping low.
Available Aerobic Power
These equations from Bassett et al.1 were generated from four groups of highly trained or elite runners, so they are population-specific to that group, but they can be used to estimate aerobic power at a given altitude as a percentage y of what is normally available at sea level, where x = elevation above sea level in km:
for acclimatized athletes (several weeks at altitude): y = -1.12x2 - 1.90x + 99.9 (R2 = 0.973)
non-acclimatized athletes (1-7 days at altitude): y = 0.178x3 – 1.43x2 - 4.07x + 100 (R2 = 0.974)
Whereas Peronnet et al.2 found
y = -0.003x3 + 0.0081x2 - 0.0381x + 1 Here is a table derived from these equations: 
Expect to see a ~1% reduction in power output per 1,000 feet of elevation gained.