The Democrats' vice presidential candidate, Virginia Sen. Tim Kaine, is reversing course on his support for the controversial Trans Pacific Partnership (TPP) a day after Hillary Clinton announced him as her running mate.

A Clinton aide confirmed to CBS News that Kaine had made a private commitment to Clinton that he would now oppose TPP, falling in line with the former secretary of state's declared view on the trade deal.

Previously, Kaine had voted for "fast tracking" authority in the Senate, which would speed trade deals like the TPP through Congress.

And of TPP itself, Kaine had seemed to defend the 12-nation trade pact in an interview with The Intercept earlier this week.

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"I am having discussions with groups around Virginia about the treaty itself. I see much in it to like. I think it's an upgrade of labor standards. I think it's an upgrade of environmental standards, I think it's an upgrade in intellectual property protections," Kaine told the publication Thursday.

But the Virginia senator had also voiced some hesitations about the trade deal.

"I do see at least right now that there is one element that I do have some very significant concerns about. And that is the dispute resolution mechanism. And I've got a lot of concerns about that," Kaine added. "But long before there would be a vote on that I'm trying to climb the learning curve on the areas where I have questions. So again, much of it I see I think as a significant improvement over the status quo. The dispute resolution mechanism I still have some significant concerns about."

Kaine is expected to publicly declare his latest views on TPP, first reported by the Washington Post Saturday, in an official announcement soon.

Clinton, for her part, fell out of lockstep with President Obama on the trade pact last year, telling CBS News that she was "not in favor" of it. Previously, the former secretary of state had advocated for TPP while she was serving as America's chief diplomat, calling it the "gold standard" of trade pacts.

GOP nominee Donald Trump, who has called the deal a "rape" of the U.S. economy, bashed Kaine on Twitter because of the senator's supportive sentiments for TPP.

Tim Kaine has been praising the Trans Pacific Partnership and has been pushing hard to get it approved. Job killer! — Donald J. Trump (@realDonaldTrump) July 23, 2016

Trump made no mention, however, of his own vice presidential pick Indiana Gov. Mike Pence, who also strongly backed the passage of TPP.

Trade means jobs, but trade also means security. The time has come for all of us to urge the swift adoption of the Trans Pacific Partnership — Governor Mike Pence (@GovPenceIN) September 8, 2014

CBS News' Steve Chaggaris contributed to this report.