Cruz is speaking on the Senate floor "until I am no longer able to stand," he said, as a protest intended to prevent Senate Majority Leader Harry ReidHarry Mason ReidHarry Reid calls on Democrats to plow forward on immigration Democrats brace for tough election year in Nevada The Memo: Biden's horizon is clouded by doubt MORE (D-Nev.) from stripping the bill of the provision that defunds ObamaCare. 

A Paul aide said that the senator, who famously launched a talking filibuster of John Brennan's nomination as CIA director to focus attention on President Obama's policy on drones earlier this year, will join Cruz on the floor to offer "reinforcements" sometime later today. It's unclear how long Paul will talk, or what he's expected to say.

Senate Minority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellBiden says he's open to altering, eliminating filibuster to advance voting rights Pelosi says GOP senators 'voted to aid and abet' voter suppression for blocking revised elections bill Manchin insists he hasn't threatened to leave Democrats MORE (R-Ky.) is opposed to Cruz's gambit.

"Senator McConnell supports the House Republicans' bill and will not vote to block it, since it defunds ObamaCare and funds the government without increasing spending by a penny," McConnell spokesman Don Stewart said Monday in a statement. "He will also vote against any amendment that attempts to add Obamacare funding back into the House Republicans' bill."

Senate Majority Leader Harry Reid (D-Nev.) is expected to advance H.J.Res. 59 with an amendment to remove the House language that defunds ObamaCare.

A vote to end debate on the motion to proceed is expected Wednesday, and Cruz's actions cannot stop that from happening.

— Ramsey Cox contributed reporting.