From the S&P news release explaining its reasoning for the downgrade:
It appears that for now,
new revenues have dropped down on the menu of policy options. In addition, the plan envisions only minor policy changes on Medicare and little change in other entitlements, the containment of which we and most other independent observers regard as key to long-term fiscal sustainability.
S&P Downgrades U.S. Debt Rating Press Release - MarketBeat - WSJ
Given the fact that the right refuses to even consider tax increases and the left is willing to allow entitlement cuts, its clear the TPMs radical no tax increase position was the primary contributing factor in enacting the downgrade.
Indeed, earlier last month Obama was willing to cut entitlements for a republican agreement to raise taxes:
[The House and Administration] are divided over taxes and entitlements. Republicans reject Democrats call for more tax revenue and instead are pressing to cut entitlement programs such as Medicare and Social Security. Democrats insist even the Republicans proposal for a smaller deficit-cutting plan must include
more taxes from higher-income Americans. Obama has said he is willing to cut entitlements in exchange for a Republican agreement to increase taxes.
Obama Said to Challenge Republicans for Debt Plan Details
And indeed the final agreement had no tax increases whatsoever.
Had an agreement been struck, per the S&P release, both increasing taxes on the wealthy and including entitlement cuts, the downgrade may very well have been avoided.
If anyone on the right has documentation of a member of the GOP leadership agreeing to tax increases in the context of an overall deal, please feel free to post it.