Sen. Mark Pryor (D-Ark.) raised $1.22 million in the last three months.
Rep. John Barrow (D-Ga.) is fighting to hold onto his House seat.
The Mississippi senator faces one of the toughest primaries in the nation.
Obama will mark the anniversary of the Boston bombings during a meeting with senior advisers.
Grimes now has almost $5 million cash on hand, while McConnell has $10.4 million in the bank.
Sink was defeated in a special election earlier this year, but Dems wanted her to try again.
The GOP-aligned group, run by Karl Rove, brought in $5.2 million in March.
New Hampshire Senate candidate Scott Brown raised $275,000 in the 16 days between the launch of his exploratory committee and the end of March, a notably strong fundraising pace for the Republican.
Rep. Tom Cotton's (R-Ark.) latest ad says Sen. Mark Pryor (D-Ark.) is "unfaithful to the truth."
Nebraska Republican Senate candidate Ben Sasse outraised his three primary opponents in the first quarter and has over $1 million in the bank heading into the final stretch of the campaign.