True the Vote declared it has evidence of tampering in runoff records.
The North Carolina Democrat raised more than $3.6 million, her campaign said.
Dems fear that a drop-off in minority voters could severely cost them at the polls this fall.
Sen. Brian Schatz (D-Hawaii) appears to have the unlikely edge in bitter race.
She said he's “being led down a primrose path to political oblivion.”
Terri Lynn Land (R) bested Gary Peters (D) with help from $1.2 million in self-funding.
President heralds vulnerable senator as "serious person who's trying to do the right thing."
The Mississippi senator's campaign will amend $53,000 in get-out-the-vote cash payments.
The Colorado Democrat said he had to stay in Washington for votes.
The Iowa GOP Senate candidate disavowed impeachment talk.