Cornyn was ranked by the National journal as the fourth-most Conservative United States Senator in their 2006 rankings. 
He is considered by the Dallas Morning News to be a reliable ally of President George W. Bush on most issues. 
Environment and energy
In 2005, Senator Cornyn voted against including oil and gas smokestacks in mercury regulations. He voted against factoring global warming into federal project planning, and against banning drilling in the Arctic National Wildlife Refuge. He voted against reducing oil usage by 40%, rather than by 5%. He also voted against removing oil and gas exploration subsidies. 
During his tenure in the Senate, Cornyn has scored 0% on the League of Conservation Voter’s environmental scorecard, a system of ranking politicians according to their voting record on environmental legislation. 
Cornyn has been described by Jim Jubak of MSN Money as one of “Big Oil’s” ten favorite members of Congress,” as he has received more money from the oil and gas industry than all but six other members of Congress. 
Civil rights and law enforcement
In September 2005, Cornyn sponsored a bill that would allow law enforcement to force anyone arrested or detained to provide samples of their DNA, which would be recorded in a central database.  He also voted for the reauthorization of the PATRIOT Act and extending its wiretap provision.
Stem cell research
John Cornyn voted against expanding research to more embryonic stem cell lines. 
War, peace, and homeland security
Senator Cornyn voted against redeploying troops out of Iraq by July 2007, and he later voted against redeploying them by March 2008. He voted against implementing the recommendations of the 9/11 Commission Report and restoring $565M for states’ and ports’ first responders. He also voted against restricting businesses with ties to terrorism. He voted against preserving habeas corpus for Guantanamo detainees. 
Economy and taxes
On January 24, 2007, Cornyn was one of 28 Senators who voted to repeal the minimum wage. 
Response to Constituents
I have written to Senator Cornyn on over 20 issues. Every time, I get back a form letter containing the sentence, “Your correspondence has been received, and we will respond to you as quickly as possible.” Only once have I ever received a followup response.