Of course he is! Who could dispute that? Thompson takes a swipe at Giuliani in a speech last night:

Presidential hopeful Fred Thompson cast himself Monday as the consistent Republican conservative in the race and suggested during a speech on Rudy Giuliani’s home turf that the former New York mayor was a liberal.
“Some think the way to beat the Democrats next year is to be more like them. I could not disagree more,” Thompson told the Conservative Party of New York.
“My friends, I suggest it’s not time for psychological flexibilities in terms of our principles. That’s the surefire way of making sure we don’t win,” he added.
The former Tennessee senator didn’t mention Giuliani, but was trying to draw a contrast with the opponent who leads in national Republican polls. Unlike Thompson, Giuliani supports abortion and gay rights. Giuliani also was once a Democrat. And, the ex-mayor’s central argument for Republicans to nominate him is that he has the best chance of winning in November 2008.
Thompson was more direct in an interview on Fox News Channel before the speech.
“I don’t think that the mayor has ever claimed to be a conservative,” he said, then rattled off Giuliani’s history.
He noted that Giuliani sought and won the Liberal Party’s endorsement in his first mayoral race in 1989, and that as a Republican mayor, he endorsed Democratic Gov. Mario Cuomo in an unsuccessful race for a fourth term.

Don’t forget guns! That anyone would buy that Giuliani is a conservative demonstrates how uninformed the voters really are.
I think it’s smart that Thompson goes after Giuliani first since he’s the biggest threat. Hannity has been pushing the fact that he’s conservative enough to get the nomination, Thompson needs to explain that he is not.