(CNN) - Sen. Scott Brown's campaign cried "hypocrite" at Elizabeth Warren on Friday over her decision not to pay the optional higher tax rate on her state return.
Warren, the Democrat who's challenging Brown for his U.S. senate seat in Massachusetts, has been a stalwart advocate for the so-called Buffett Rule–a proposal advanced by the Obama administration that calls on people making more than $1 million a year to pay federal taxes amounting to at least 30% of their income.
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Senate Republicans, including Brown, blocked the measure earlier this week.
Because Warren, a former Obama adviser who paints herself as the middle class advocate, repeatedly pushes for the wealthy to pay a "fair share" in taxes, Brown's team argues that she should have no problem paying higher taxes, given her six-figure salary and millions in assets.
"The problem with running a campaign based on self-righteousness and moral superiority is that you had better live up to the same standard you would impose on everyone else," Jim Barnett, Brown's campaign manager, said in a statement.
Very few taxpayers choose to pay the optional rate. According to Bob Bliss, communications director at the state's revenue department, about 650 out of 3.5 million people selected the rate this year.
The state's optional tax rate became effective in 2001 as part of a compromise for those who opposed the state's voter-approved tax cuts the previous year.
Massachusetts is one of eight states with so-called Tax Me More Funds, according to the Americans for Tax Reform Center for Fiscal Accountability.
Warren's campaign confirmed the candidate opted out of paying the state's elective tax rate of 5.83%, which is slightly higher than the normal tax rate of 5.3%.
Her spokeswoman, Alethea Harney, argued the point of the Buffet Rule is for everyone to play by the same rules, not arbitrary or voluntary rules.
"The Buffet Rule is about making sure millionaires and billionaires pay their fair share, not about funding government through voluntary contributions. This question misses the key point in the debate, because it's not just about economics but about our values," she told CNN.
Brown, as expected, also did not pay the optional tax rate.
Warren and Brown are locked in a tight Senate race, as Brown fights to keep the seat he won in a 2010 special election to replace the late Democratic Sen. Ted Kennedy who held the seat for decades. Polls in recent months have indicated Brown and Warren remain statistically tied.
In terms of fundraising, however, Warren has far outpaced her opponent, raising a mammoth $6.9 million in the first quarter, compared to Brown's $3.4 million.
Despite her fundraising advantage in the first three months of the year, Brown has $4 million more cash on hand heading into the 2012 election, at $15 million.