100%

If there was anything resembling message discipline in the Democratic Party, this statistic would be on the lips of every "strategist" and surrogate, in every media appearance, for the next five months.

CQ’s Presidential Support studies try to determine how often a legislator votes in line with the President’s position:

CQ tries to determine what the president personally, as distinct from other administration officials, does and does not want in the way of legislative action. This is done by analyzing his messages to Congress, news conference remarks and other public statements and documents.

So, these studies only track votes when the President has an explicit, stated opinion on a bill.

According to CQ, Senator John McCain has voted with President Bush 100% of the time in 2008 and 95% of the time in 2007.

Sure, it doesn't include votes that legislators miss - so what, neither does the National Journal's "Obama's the most liberal member of the US Senate" rating, that doesn't stop conservative commentators from trotting that one out. And I'd say this one is more impactful - it's clear when Bush takes a position on a bill, there's no shading or gradation there, unlike in NJ's ratings, where they make value judgments on what votes are "liberal" or "conservative". And every time he took such a position in 2008, John McCain voted the same way.

McCain may not want to be seen with George Bush. But the record is harder to run from. He's exactly the same.