(CNN) – Sen. John McCain seems to be confirming his "maverick" nickname this week.

The Vietnam war hero, longtime Arizona senator and 2008 Republican presidential nominee made headlines in recent days both for bucking members of his own party and for staunchly defending the current GOP nominee against attacks. And Thursday, he pushed back against both parties for their reliance on big dollar donors who he says are corrupting the American political system.

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"I promise you there will be scandals, and there will be major scandals," McCain told CNN's Piers Morgan in an interview set to air Thursday at 9 p.m. ET. "There's too much money washing around for there not to be scandals. If you think Watergate was big, if you think Abramoff was big, just wait and you'll see what happens in this thing."

McCain has long advocated campaign finance reform, including sponsoring the McCain-Feingold legislation in 2002 that overhauled campaign spending laws. Since then courts have struck down various aspects of that measure.

Earlier this week McCain voted with fellow Senate Republicans to block the DISCLOSE Act, a piece of Democratic legislation that would require greater transparency into who is behind much of the secretive, often negative campaign advertising filling the airwaves this election season.

McCain said he was opposed to the bill because it had different rules for labor unions, which he said were getting off easy.

Later in the week, however, McCain hit back against members of his own party, including Minnesota Rep. Michele Bachmann, for their requests that various government agencies investigate whether the government has been infiltrated by Muslim extremists.

Among the issues they raise is a claim that long-time aide to U.S. Secretary of State Hillary Clinton, Huma Abedin, has three family members connected to the Muslim Brotherhood, and is at risk of being influenced by her family members. The members of Congress want to know how she holds a high level security clearance.

McCain made a speech on the Senate floor Wednesday criticizing the call for an investigation and defending Abedin.

Asked by Morgan if the floor speech meant "we've got the real John McCain back," McCain said he'd argue "I never left."

"The point is that I know Huma. I know her from travelling with her, I know of her reputation, I know this great story that we love - the immigrant who comes to, you know, to our country and does so well. And it was really unfair to do what was done to her," McCain said, adding he wasn't only targeting Bachmann in his speech.

"I hope that this sends a message to other people on both right and left that we shouldn't attack people's character unfairly," McCain said.

Bachmann, who refused to answer questions from CNN about the letter on Wednesday, said in a statement that her and her colleagues' request was being "distorted."

"I encourage everyone, including media outlets, to read them in their entirety," Bachmann wrote. "The intention of the letters was to outline the serious national security concerns I had and ask for answers to questions regarding the Muslim Brotherhood and other radical group's access to top Obama administration officials."

McCain also finds himself in the unique position of being one of the few Americans who has seen more than two years of Mitt Romney's income tax returns. Romney, who made his own bid for the 2008 GOP presidential nomination, reportedly provided 20 years of tax returns to the team vetting potential vice presidential picks for McCain. That's significantly more than the two years of information Romney has made public this cycle.

Romney has released his 2010 tax returns and an estimate for 2011, with a promise to release the official documents for last year once they're ready. He filed an extension with the Internal Revenue Service, meaning he may not put out his returns until the October filing deadline.

Democrats, in their effort to get Romney to release more tax returns, now ask why Romney was willing to give so many documents to McCain, but won't do the same publicly this year. Some have questioned whether McCain saw something in Romney's tax information four years ago that discouraged him from picking Romney as his running mate.

McCain says that's not the case.

"I can personally vouch for the fact that there was nothing in his tax return that would in anyway be disqualifying for him to be a candidate. Okay?" McCain said Wednesday.

"We're getting into areas now that are improper for me to talk about," McCain added. "Because when people gave us all those records we agreed to confidentiality. The only reason I'm saying what I'm saying now is because of the scurrilous, scurrilous Chicago style sleaze intimations with no basis in fact that there might have been something wrong with his tax returns, which is disgraceful."

CNN's Ashley Killough and Ted Barrett contributed to this report.