Warren's campaign has worked to tie Brown to the GOP, saying he's backed tax breaks for oil companies and has spoken out against a proposal by Obama to require people making more than $1 million a year to pay at least 30 percent in taxes.
"Scott Brown's endorsed Mitt Romney, not President Obama, and on important issues — the Buffet rule, jobs bills, and tax breaks for oil companies — Scott Brown has turned his back on Massachusetts and on President Obama to stand with and vote for the interests of Wall Street, big banks and big oil," said Warren press secretary Alethea Harney.
Brown has said he's just showing what he says is his independent streak by looking each issue individually and deciding what he thinks is best for Massachusetts and the country, regardless of party affiliation.
That emphasis on Brown's "independent voice" could be critical in November.
While Massachusetts tends to elect Democrats, the majority of voters in the state are not registered with either party.
It was by appealing to those independent voters, and picking off moderate Democrats, that Brown was able to win in 2010. While that campaign was compressed into a few weeks — allowing Brown to ride a wave of popularity — the current campaign will play out between now and November.
There are limits to Brown's bipartisanship.
During a campaign kick-off rally in Worcester, Mass., Brown — speaking to a more partisan crowd — only invoked Obama's name when he called for the repeal of what he called "Obamacare."
"One unfinished fight is the effort to get rid of Obamacare. A government takeover of health care was a bad enough idea to start with, and the way it was rammed through was even worse," he told the cheering crowd. "I was against Obamacare then, and I am for its repeal today."
Massachusetts Democrats have struggled to find a way of targeting Brown as he highlights areas of agreement with the president.
As Brown spotlighted Obama's support of his anti-insider trading bill, Democrats said Brown should go a step further and back another Obama proposal to limit any elected officials from owning stocks in industries affected by their votes.
"Scott Brown should join President Obama and Senate Democrats' call to ban members of Congress from owning stock in companies they influence if he is really serious about cleaning up Washington," said Massachusetts Democratic Party spokesman Kevin Franck.
The Senate campaign could end up being the most expensive in Massachusetts history.
Last month, Brown and Warren took the unusual step of signing a pledge to curb political attack ads by outside groups in their Massachusetts Senate race. Under the terms of the deal, each campaign would agree to donate half the cost of any third-party ad to charity if that ad either supports their candidacy or attacks their opponent by name.
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