Sen. Joe Lieberman (ID-Conn.) has resumed his role as an attack dog for Sen. John McCain (R-Ariz.) by accusing Democratic presidential hopeful Sen. Barack Obama (Ill.) of putting the needs of other nations ahead of the United States and arguing that he is unfit to lead the nation during wartime.

Although Lieberman’s comments prompted no direct public reaction from either Obama’s campaign or top Democratic leaders, Democrats privately said Lieberman’s comments could cost him in the Senate, where he caucuses with Democrats.

One senior Senate Democrat acknowledged that Majority Leader Harry Reid (D-Nev.) and other leaders continue to keep an eye on Lieberman.

Reid has already warned Lieberman once this year that Democrats were unhappy with his direct attacks on Obama. Lieberman then seemed to back off from attacking Obama, saying in June that “Obviously I support Sen. McCain. … But I can only do so much as long as it doesn’t interfere with what I’m doing here.”

Lieberman’s comments Tuesday seemed to ratchet up the tone.

At a town hall meeting in Pennsylvania and again during a fundraising event for McCain in New Jersey, Lieberman launched blistering attacks on Obama’s ability to lead.

Pointing to the two candidates’ approach to the Russian invasion of Georgia, Lieberman said at the town hall event that “the choice could not be more clear between one candidate, John McCain, who’s had experience, been tested in war and tried in peace, [and] another candidate who has not — between one candidate, John McCain, who’s always put the country first, worked across party lines to get things done, and one candidate who has not.”

He added the race boiled down to “one candidate who’s a talker and the other candidate who’s the leader America needs as our next president. … We never know when we elect a president what crisis will occur in the four years of his watch.”