President Obama is confident that a sufficient number of Senate Republicans will end up supporting ratification of the START treaty.

Obama signed the bilateral U.S.-Russia arms treaty in Prague today, leaving it up to the Senate to muster the 67 votes needed to ratify it.

The president told ABC News that the GOP will come around.

"When they have had the opportunity to fully evaluate this treaty, they will come to the conclusion that this is in the best interest of the United States," Obama told George Stephanopoulos.

He added: "I will also say to those in the Senate who have questions is that this is absolutely vital for us to deal with the broader issues of nuclear proliferation, that are probably the number one threat that we face in the future."

The treaty would end up reducing the nuclear stockpile of both the United States and Russia by about a third.

Sen. Richard Lugary (R-Ind.), ranking member of the Senate Foreign Relations Committee, reportedly feels "favorable" about the treaty, but hasn't yet committed to supporting it.