McCain's record in Congress: Not great
During his thirty years in Congress as a representative and senator, John McCain sponsored about 773 bills. Seventeen were passed into law. The most notable bills he sponsored are the Climate Stewardship Act, with Joe Lieberman, which failed to pass; the Bipartisan Campaign Reform Act, also known as McCain-Feingold; and the Detainee Treatment Act. Both of the last two passed.
George W. Bush believed that parts of McCain-Feingold, which restricted free speech, was unconstitutional, but he signed it, saying the Supreme Court would resolve the issues.
Bush did not uphold his duty to protect the Constitution.
The Supreme Court in Citizens United v. FEC found key parts of McCain-Feingold unconstitutional because it restricts free speech.
McCain's other accomplishments as a "maverick" are that he voted against the Bush 2001 and 2003 tax cuts and voted against the Trump repeal of Obamacare.
McCain also was a sponsor with Ted Kennedy of the 2007 McCain-Kennedy "Comprehensive Immigration Bill," which failed to pass after the Republican base revolted against it.
McCain styled himself as a "maverick," but his record as a maverick was to oppose Republicans. This endeared him to the Dems and media, except in 2008, when McCain ran against Obama.
McCain used his record as a POW to launch his political career, and he used it throughout his political career.
McCain's record does not show any significant legislation for conservative causes, nor as a leader to unite Republicans against Democrats. He saw himself as "reaching across the aisle," which meant sponsoring bills with Teddy Kennedy and Russ Feingold that furthered the agenda of Democrats.
McCain's final thumb in the eye to conservatives was to have Joe Biden give his eulogy. Biden led the attack to destroy Justice Clarence Thomas. Biden also joined with McCain's pal, Teddy Kennedy, to destroy Judge Robert Bork. Both Biden and Teddy Kennedy defamed Justice Thomas with vicious, unfounded personal attacks. McCain's other thumb in the eye was his "thumbs down" on voting against repeal of Obamacare.
McCain had Feingold, Sheldon Whitehouse, Gary Hart, Warren Beatty, and Michael Bloomberg as pallbearers. Not one of them voted for McCain in 2008.
John Kerry offered the V.P. slot to McCain in 2004.
McCain should have been honest and run as Democrat. He was more comfortable working with the likes of Teddy Kennedy, Russ Feingold, Joe Biden, and John Kerry than with Republicans, especially conservatives.