Sen. Rand Paul's (R-Ky.) filibuster is now officially bipartisan.

Sen. Ron Wyden (D-Ore.), a libertarian-minded Democrat who has also expressed reservations about the Obama administration's drone policy, shortly before 4 p.m. became the fifth senator and first Democrat to join Paul's filibuster.

"It's my view that the senator from Kentucky has made a number of important points today," Wyden said.

“I want it understood that I have great respect for this effort to really ask these kinds of questions," Wyden said. "And Sen. Paul has certainly been digging into these issues in great detail."

At the same time, Wyden said he was satisfied with Attorney General Eric Holder's response to Paul's question about drone strikes on American soil.

Four Republicans have joined the filibuster as well: Sens. Mike Lee (R-Utah), Ted Cruz (R-Texas), Jerry Moran (R-Kan.) and Marco Rubio (R-Fla.).

Updated at 4:19 p.m.