Republican Sen. Kelly Ayotte of New Hampshire said today that she will support the immigration reform bill drafted by the bipartisan "Gang of Eight," arguing that the legislation provides a "tough but fair way" for undocumented immigrants to earn citizenship.

"This is a thoughtful bipartisan solution to a tough problem, and so that's why I'm going to support it," Ayotte said today on CBS' "Face the Nation."

Ayotte's support adds to the small tally of Republicans currently promoting the bill, including the four GOP senators in the "Gang of Eight." However, one of the bipartisan group's members, Sen. Marco Rubio, R-Fla., has threatened to vote against the bill unless it includes tougher border security provisions.

In an interview on Univision's "Al Punto," Rubio argued that strengthening the border security measures would help "earn our colleagues' trust" and predicted that his group will find enough votes to exceed the 60 required to prevent a filibuster.

"We'll have a lot more than 60 votes, but we're going to have to work at it," Rubio told Univision's Maria Elena Salinas in an interview that aired today on "Al Punto."

Sen. Rand Paul, R-Ky., said today he "is willing to compromise" on the immigration plan if it includes changes like tougher border measures, and he suggested he could serve as a "conduit" to House conservatives who currently disagree with the plan proposed in the Senate.

"I am the conduit between conservatives in the House who don't want a lot of these things and more moderate people in the Senate who do want these things. I want to make the bill work, but see, the thing is, is what they have in the Senate has zero chance of passing in the House," Paul said on "FOX News Sunday." "I'm really trying to make immigration work, but they're going to have to come to me, and they're going to have to work with me to make the bill stronger if they want me to vote for it."

Formal debate on the bill started Friday in the Senate and is expected to continue through the week.