WASHINGTON, DC - MARCH 21: U.S. Sen. Heidi Heitkamp (D-ND) speaks during a hearing before the Senate Homeland Security and Governmental Affairs Committee March 21, 2013 on Capitol Hill in Washington, DC. The committee held a hearing to get a progress report on the management of the Department of Homeland Security 10 years after its creation. (Photo by Alex Wong/Getty Images) Sen. Heidi Heitkamp (D-N.D.) (Alex Wong/Getty Images)

A sixth Senate Democrat announced her opposition Monday to authorizing President Obama to use military force in Syria.

Sen. Heidi Heitkamp (D-N.D.) said in a statement that she will continue to listen to the case for intervention, but that she would be a "no" vote if the vote were held now.

“I still believe we need to have an open and honest discussion on the Senate floor about the potential use of force in Syria," Heitkamp said. "However, after all these meetings, I still have serious concerns. I cannot support the current Senate resolution to authorize force at this time."

The other Senate Democrats who have expressed opposition are Sens. Brian Schatz (D-Hawaii), Tom Udall (D-N.M.), Chris Murphy (D-Conn.) Joe Manchin (D-W.Va.) and Mark Pryor (D-Ark.).

With many Republicans expressing opposition — especially in the House — the Obama administration increasingly needs to keep the Democratic Party united to pass the resolution through Congress. To this point, though, significant portions of the Democratic caucuses in both the House and Senate are opposed.

While the House remains the big hurdle for Obama, the Senate is looking increasingly difficult as well.

Thirteen Senate Republicans have expressed opposition, while nine more senators from both parties are leaning against voting for the use of force.

The Fix's latest whip count shows 23 senators favor military action. Nearly half the Senate — 48 members — remains undecided.

Update 1:36 p.m.: Sen. Lamar Alexander (R-Tenn.), who had been leaning against military action, said Monday he will vote 'no.' This post has been updated to reflect this.

"I will vote 'no' because of too much uncertainty about what comes next," Alexander said, according to the Memphis Commercial-Appeal. "After Step A, what will be Steps B, C, D and E? I see too much risk that the strike will do more harm than good, by setting off a chain of consequences that could involve American fighting men and women in another long-term Middle East conflict."