Those backers are referring to this vote, taken in June 2008. As you can see from the roll call, fully 24 Republicans voted to impeach President Bush, including conservatives like Pete King of New York and Kevin Brady of Texas. One imagines that would have been rather big news had it happened.
But it didn’t happen. The vote in question was on a motion to refer Rep. Dennis Kucinich’s impeachment resolution to the Judiciary Committee, where most of the Republicans who voted on it calcluated it would die a slow death, with no one willing to give it time in the committee.