Sen. Kelly Ayotte (R-N.H.) on Sunday said she will support the immigration reform bill proposed by the bipartisan Senate "Gang of Eight."

"I have looked at this carefully," Ayotte said on CBS's "Face the Nation." "This is a thoughtful bipartisan solution to a tough problem. And so that’s why I’m going to support it."

Ayotte's support is significant because it means the bill -- as long as it gets the support of all or most of the 54 members of the Democratic caucus in the Senate -- is approaching the 60 votes it would need to override a filibuster.

That's assuming, of course, that in addition to Ayotte it gets the support of the four Republicans who belong to the Gang of Eight. But one of them -- Sen. Marco Rubio (R-Fla.) -- has suggested he won't support the bill unless it includes more border security.

A sixth Republican, Sen. Orrin Hatch (R-Utah), voted for the bill in the Senate Judiciary Committee.

On the Democratic side, Senate Majority Leader Harry Reid (D-Nev.) said last week that the bill was supported by at least 90 percent of Senate Democrats.

The bill made its way last week to the Senate floor, where members are set to offer amendments.

Update 11:40 a.m.: Ayotte has sent a lengthy e-mail to supporters and written an op-ed explaining her decision.