Members of the Senate are rushing for the exits in the wake of the Internet's unprecedented protest of the Protect IP Act (PIPA). At least 13 members of the upper chamber announced their opposition on Wednesday. In a particularly severe blow for Hollywood, at least five of the newly-opposed Senators were previously co-sponsors of the Protect IP Act. (Update: since we ran this story, the tally is up to 19 Senators, of which seven are former co-sponsors. See below.)

The newly-opposed Senators are skewed strongly to the Republican side of the aisle. An Ars Technica survey of Senators' positions on PIPA turned up only two Democrats, Ben Cardin (D-MD) and Jeff Merkley (D-OR), who announced their opposition on Wednesday. The other 11 Senators who announced their opposition on Wednesday were all Republicans. These 13 join a handful of others, including Jerry Moran (R-KS), Rand Paul (R-KY), Mark Warner (D-VA), and Ron Wyden (D-OR), who have already announced their opposition.

Marco Rubio, a freshman Republican Senator from Florida who some consider to be a rising star, withdrew his co-sponsorship of the bill, citing "legitimate concerns about the impact the bill could have on access to the Internet and about a potentially unreasonable expansion of the federal government's power to impact the Internet." He urged the Senate to "avoid rushing through a bill that could have many unintended consequences."

Another co-sponsor, Sen. Roy Blunt (R-MO) echoed that sentiment. He blamed Senate Majority Leader Harry Reid (D-NV) for "pushing forward w/ a flawed bill that still needs much work."

Sen. Orrin Hatch (R-UT), one of the chamber's longest-serving members and another co-sponsor, described the Protect IP Act as "simply not ready for prime time."

The partisan slant of the defections is surprising because copyright has not traditionally been considered a partisan issue. Before Wednesday's protests, PIPA had 16 Republican co-sponsors and 23 Democratic ones. The bill lost a quarter of its Republican co-sponsors on Wednesday, while we know of only one Democrat, Ben Cardin (D-MD), who dropped his support.

Those who dropped their support were most likely bolstered by strong opposition from conservative think tanks and blogs. On Tuesday, the influential Heritage Foundation announced that it would include SOPA and PIPA as a key issue on its voter scorecard. And the popular conservative blog redstate.com, whose founder threatened to mount primary challengers to SOPA supporters last month, has been hailing Senators who come out in opposition.

Neither side is close to having a majority. A whip count by OpenCongress found 35 supporters (including 34 co-sponsors), 18 opponents, and 12 more Senators leaning toward opposition. About 35 Senators have not committed to a position, perhaps reluctant to do so for fear of angering either deep-pocketed Hollywood campaign contributors or their constituents back home.

Here is the full list of new opponents. An * indicates a former co-sponsor.

Update (7 PM): David Vitter (R-LA) is now also opposed. He was previously a co-sponsor of the legislation.

Update (9 PM): Three more opponents, all Republicans: Tom Coburn (R-OK), Pat Toomey (R-PA), and Mike Johanns (R-NE).

Update (10 PM): Kelly Ayotte (R-NH) is jumping on the bandwagon. She was a PIPA co-sponsor.

Update (Thursday): Mark Begich (D-AK) also joined the opposition on Wednesday.