Sen. Marco Rubio, the leading Republican behind the Gang of Eight comprehensive immigration reform bill, says he will not vote for the legislation he helped write and has staked his political future on, unless substantial changes are made before final Senate consideration.

Speaking with radio host Hugh Hewitt Tuesday, Rubio said the Senate should “strengthen the border security parts of this bill so that they’re stronger, so that they don’t give overwhelming discretion to the Department of Homeland Security.” He said he was working with other senators on amendments to do just that.

Then Hewitt asked: “If those amendments don’t pass, will you yourself support the bill that emerged from Judiciary, Senator Rubio?”

Rubio answered, “Well, I think if those amendments don’t pass, then I think we’ve got a bill that isn’t going to become law, and I think we’re wasting our time. So the answer is no.”

Rubio’s turning on his own bill would be an extraordinary turn of events. After playing a major role in drafting the legislation, Rubio has been its public face since then, making countless appearances on television, radio, and in print to gather support for the legislation. What has changed that would mean he would not vote for his own bill? If anything, the security measures in the bill were slightly strengthened in the Senate Judiciary Committee; the bill’s original intention to apply new security provisions only to “high-risk” sectors of the U.S.-Mexico border was expanded to apply to all sectors.

So it would be hard to argue that the Judiciary Committee changed the bill in ways that would make it unacceptable to Rubio. But now he says he will vote against the bill approved by the committee, unless major changes are made. It is hard to tell if Rubio really disagrees with the substance of the legislation approved by the committee — he suggested to Hewitt that he would vote against the bill because it wouldn’t pass without the changes, not because he objected to particular passages in the bill — but the result would be that the principal author of comprehensive immigration reform would vote against it in the Senate