Sen. Tim Kaine (D-Va.) on Capitol Hill on Feb. 3. (J. Scott Applewhite/AP)

Sen. Rand Paul gained an important ally Tuesday in his quest to have the Senate vote to force a debate on authorizing the military’s combat operations against extremist groups, as Sen. Tim Kaine signaled that he was ready to come on board.

But Kaine’s decision to join Paul’s efforts did not inspire other senators to follow suit, as leading Republicans and Democrats argued against putting an expiration date on existing authorizations for the United States’ engagement in high-stakes conflicts around the world.

Kaine (D-Va.), who has been one of the Senate’s most consistent voices urging Congress to weigh in on the military’s engagement against extremist forces in Iraq, Afghanistan and elsewhere, said Tuesday that he would support Paul’s (R-Ky.) amendment if it comes up for a vote.

“I view his amendment as an attempt to force Congress to do what it should do,” Kaine said. “I think it is way past time, way past time, for Congress to take this up and for everybody to be on the record.”

It's a change of heart for Kaine, who was quick to criticize last week when Paul launched his effort to add to the defense bill a six-month deadline to pass an authorization for military force. He said Paul's campaign would "unnecessarily" hold up progress on the defense bill and that such a delay would "be a disservice to our service members and their families." He added that the Senate Foreign Relations Committee was the proper forum in which to debate an Authorization for Use of Military Force (AUMF).

But Tuesday, Kaine’s patience appeared to have waned. He announced on the floor that he is “supporting Senator Paul’s amendment,” because there was “no particular motive or forcing mechanism that has made the committee take this up.”

Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.), who has promised that his panel will tackle an AUMF proposal soon, is not expected to object to Paul’s amendment coming up for a vote.

Kaine’s turnaround was an important boost for Paul, who often has been on his own in advocating isolationist positions. Paul was alone Monday in threatening to block progress on the defense bill until lawmakers tackle his AUMF amendment.

But others were quick to denounce the effort as one that could jeopardize national security.

“We have to think seriously about what the message would be if we adopted this resolution,” said Sen. Jack Reed (D-R.I.), the ranking member on the Senate Armed Services Committee. “The headlines in Baghdad and the headlines in Kabul and the headlines in Damascus would be ‘U.S. moves to end engagement.’ . . . Unless we could do something literally next week, we would be running into the reality of American military commanders wondering whether or not they should begin to plan for the extraction of our forces.”

A one-week turnaround is a fanciful idea for a Congress that has unsuccessfully grappled with AUMF debates for years.

Kaine is an author of the AUMF proposal that has featured most prominently in that debate, along with Sen. Jeff Flake (R-Ariz.). Their proposal would replace the 2001 AUMF, which Congress passed to greenlight the war in Afghanistan, with a new measure focused on fighting extremist groups such as al-Qaeda, the Islamic State and the Taliban.

Flake split from Kaine on Tuesday, stating on the Senate floor that he could not support Paul’s amendment “because of the very real risk associated with repealing such a vital law before we have something to replace it with.”

A spokeswoman for Senate Armed Services Committee Chairman John McCain (R-Ariz.), who is in charge of the defense bill’s progress through the Senate, would not say Tuesday whether leaders were planning on allowing a vote on Paul’s amendment.

Practically speaking, Paul’s ability to block progress on the defense bill is limited: He can slow the debate to advocate for his AUMF measure, but he cannot indefinitely prevent the defense bill from advancing.

The Senate is scheduled to take its next procedural vote on the defense bill at 10 a.m. Wednesday.