National Republican Senatorial Committee Chairman John Ensign (Nev.) said Monday afternoon that it was a “fair possibility” that Democrats could reach a filibuster-proof, 60-seat majority in the Senate on Nov. 4.

Speaking to Andrea Mitchell on MSNBC, Ensign seemed to upgrade the Senate Democrats’ chances for widening their margins. Democrats currently hold a 51-49 majority in the chamber.

As recently as last week, Ensign suggested Democrats were likely to increase their numbers to 57 seats.

Republican Sens. Norm Coleman (Minn.), Elizabeth Dole (N.C.) and Gordon Smith (Ore.) are among a growing list of lawmakers that are now in tossup races, with just eight days to go before Election Day.

Ensign said the national surge in popularity of Democratic presidential hopeful Sen. Barack Obama (Ill.) is starting to hurt Republican candidates down the ballot.

“It’s no question that the top of the ticket is affecting our races. It’s a lot more difficult,” the Nevada Republican said.

Ensign did show some optimism, however. He suggested Sen. Joe Lieberman (ID-Conn.), who caucuses with the Democrats but has aligned himself with Arizona GOP Sen. John McCain’s presidential campaign, might consider switching parties next year. Lieberman has angered Democratic Senators for his ardent support of McCain and attacks against Obama.

“We would welcome Joe Lieberman. We welcome him with open arms,” Ensign said.