"There's always been an anti-Mitt Romney movement and Newt Gingrich has now captured it, but Republicans on Capitol Hill are fearing, many of them, that Gingrich will actually win the nomination," said Cusack.
Cusack said concerns over Gingirch's leadership abilities during his time as Speaker of the House have many lawmakers scrambling to endorse Romney.
Romney has secured dozens of endorsements from congressional lawmakers, while Gingrich only has around eight.
For up-to-date coverage of 2012 GOP lawmaker endorsements for president from The Hill, click here.
"They didn't like how he consolidated the power within the Speakership and they just didn't like his way about him. They didn't think he managed the House well, and they say, 'Listen, if you can't lead the House, how can you run the White House?' " said Cusack.
Cusack told "Focus Washington" that despite concerns raised by former House colleagues, Gingrich has become a "real threat" to Romney's candidacy.
"Will it last? I don't know. But he's got strong leads in these early states and Mitt Romney, there's always that 'anybody but Mitt' movement," added Cusack.
However, Cusack said, for Gingrich to continue his lead in the GOP field he will need to raise more money and improve his campaign organization. Romney already has plenty of money and strong organization.