An old nemesis has a message for Scott Brown as he prepares to tackle a new foe.

US Senator Elizabeth Warren today said the man she unseated in 2012 should be prepared for a hard fight against US Senator Jeanne Shaheen of New Hampshire.

“Jeanne is tough, she is independent, and she is strong,” Warren told the Globe after attending a City Hall press conference in Boston.

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“This will be up to the people of the Commonwealth... whoops, this will be up to the people of New Hampshire,” Warren said, laughing. “But I think Scott Brown is going to have his hands full.”

Asked if she was confident Shaheen would be victorious, Warren replied, “Yes. Oh, yes!”

Brown, who has been exploring a bid in the Granite State, this morning said he would make his US Senate run official at a rally on Thursday in Portsmouth.

In an e-mail to supporters, Brown emphasized his ties to the state where he spent his early childhood.

“Portsmouth played a special role in my childhood. I remember our house on Islington Street, strolling through Strawbery Banke with my grandfather, and catching a show at Prescott Park,” he wrote.

“Now, Portsmouth is going to be the start of the next chapter in my life,” he added, inviting supporters to join him at his kickoff event.

Brown, who won an upset 2010 special election victory over Martha Coakley, lost to Warren in 2012 and moved his primary residence to New Hampshire late last year.

He will face a GOP primary race before he can go head-t0-head with Shaheen. Other Republican contenders include former US senator Bob Smith, former state senator Jim Rubens, and conservative activist Karen Testerman.