Vice President Biden added a "caveat" on Friday to his frequent predictions that Democrats would keep control of the House and Senate in Nov. 2's elections.

The vice president said that the heavy spending by outside groups on behalf of Republicans in the closing weeks of the campaign could alter the electoral playing field to the extent that control of the House might be in play.

“The Republicans will gain seats. We will keep control of the Senate for certain, and I believe we'll keep control of the House," Biden said in an interview to air over the weekend on Bloomberg's "Political Capital With Al Hunt."

"I've never seen this before, so the only caveat I'd put in terms of the House is how much impact this $200 [million is] going to mean," Biden added, referring to the spending by outside groups on the election.

Biden, President Obama and other Democrats have railed against the U.S. Chamber of Commerce and American Crossroads, a GOP group affiliated with former George W. Bush administration officials Karl Rove and Ed Gillespie, for spending millions on the election, fueled by unlimited donations.

But the administration has generally expressed confidence about its ability to stave off the net loss of 39 seats in the House and 10 seats in the Senate, which would allow the party to keep control of both chambers.

Biden's been known to paraphrase the writer Mark Twain on the campaign trail, telling audiences at nearly 100 events this year that rumors of the Democratic Party's death have been greatly exaggerated.

And the vice president has frequently lobbed attacks at the GOP, warning, like other Democrats, that Republican victories on Election Day would mean the revival of unpopular policies.

“I don't get them," Biden said. "I don't understand why they really think that what they're proposing can do anything other than drive us further in the ditch.”