It definitely would! The reason is simple: Japan!
When comes to conflict between China and Japan, the US always side with Japan, why? Tactical strategy.
If China was allied with US, then the US would definitely change its tactics when dealing with China-Japan conflict. Most likely the US will remain neutral.
Maybe if they allied along with Canada and Russia we could take over the world, therefore spreading democracy to all corners of the Universe. Not just the world the Universe. Spreading the message of capitalism through, the most communist countries. Not even YOU could stop it Fan7651230. No one, no one, NNNOOO OOONNNEEE.
I think that we would be stronger partners against Japan during and after WW2. We would later become allies just like we became allies with Japan and Germany (with out the reconstruction part of course) and would possibly still be allies or have close relations today. That is my opinion.
The U.S and Allies would definitely see China as a superpower(military-wise) and would eagerly accept China as an ally. With the U.S as an ally, China could possibly integrate Taiwan, cripple Japan(economically, or physically), and possibly boost their economy as well. Also with U.S as an ally, Sino-American relations would significantly increase, allowing more American businessmen and companies to invest and start a end to China's cheap labor. Thus its a win-win situation for both the U.S and China.
The Americans and the Chinese have one of the biggest economies of the world and their economies are entwined but as it goes on now its all fine with U.S having the upper hand and China rapidly closing in on the gap but if a sudden shift in tide favoring either of these countries occurs then problems may occur between them and as all of us can see U.S in helping the countries in East China sea to prevent their lands or maritime interests to be bullied by China.All this rises the stakes.Pakistan on one side has seen a potential ally in China which was held by U.S some time before and anything that Pak or China does that offends American policies in that area could have adverse affects with India largely alone in that area U.S will be keen on supporting India as their relations are growing rapidly so are China's and Pakistan's.Remember South Asia is very fragile with three main nuclear powers two arch enemies India and Pakistan and the other a potential superpower.
And of course two superpowers will always have a minor problem which will always be exaggerated calling for trouble...
They could be friends if South Asia and the East China sea remains calm...Which is highly unlikely...
China sees itself as the only competitor on the world stage with the USA. Its great size and perceived economic strength give the Chinese the idea that they should be the international watchdog, not the USA. China is already a world and regional power, with heavy influence over its neighbours, and many developing countries around the world it has invested in. By allying with the West, it makes itself less of a global player and appear weak and dependent on the West. Animosity between China and Japan from WW2 is still simmering, and since Japan is a major ally of the USA, this would present another barrier between an alliance.
An example of this can be seen in Russia. When the USSR collapsed, everyone thought that Russia was going to become more Western and democratic, possibly even allied with the USA. Instead, they have gone completely separate paths. Russia, fearful of losing their remaining influence in the world, have engaged in a series of quasi-wars against the West.
China won't let go of its ambitions easily.