Eight Democrats have topped $1 million raised in the last three months. Sen. Al FrankenAl FrankenDems press Trump to keep Obama overtime rule GOP wants to move fast on Sessions Overnight Cybersecurity: Lawmakers pushing for vote to delay warrant rule changes MORE (Minn.) leads the pack so far with just under $2 million raised, with Sen. Mark PryorMark PryorCotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm Top Democrats are no advocates for DC statehood MORE (Ark.) close behind after a $1.9 million quarter. Sen. Kay HaganKay HaganGOP senator floats retiring over gridlock 2016’s battle for the Senate: A shifting map The untold stories of the 2016 battle for the Senate MORE (N.C.) brought in $1.6 million, Sen. Mark UdallMark UdallGardner's chief of staff tapped for Senate GOP campaign director The untold stories of the 2016 battle for the Senate Colorado GOP Senate race to unseat Dem incumbent is wide open MORE (Colo.) raised $1.5 million, and Sens. Mary LandrieuMary LandrieuFive unanswered questions after Trump's upset victory Pavlich: O’Keefe a true journalist Trump’s implosion could cost GOP in Louisiana Senate race MORE (La.) and Jeanne Shaheen Jeanne ShaheenThis Week in Cybersecurity: Dems press for information on Russian hacks Overnight Tech: Venture capitalists' message to Trump | Bitcoin site ordered to give IRS data | Broadband gets faster Dem senator: Hold hearing on Russian interference in election MORE (N.H.) brought in $1.2 million apiece. Landrieu has just shy of $3.5 million in the bank for her reelection push.

Early money is key for these candidates, as many are likely to face well-funded challengers and Republican-affiliated outside groups have a big cash advantage. Candidates also get air time at cheaper rates than committees and outside groups, making their money go further. 

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One caveat: Sens. Max BaucusMax BaucusThe mysterious sealed opioid report fuels speculation Lobbying World Even Steven: How would a 50-50 Senate operate? MORE (D-Mont.) and Mark BegichMark BegichThe future of the Arctic 2016’s battle for the Senate: A shifting map Trump campaign left out of Alaska voter guide MORE (D-Alaska) haven't posted their fundraising totals, and candidates with lower numbers often wait until the last minute to put their numbers out. Baucus already has $3.6 million in the bank, however, money that goes far in the inexpensive state. Begich had just shy of $1 million as of the end of 2012.

Another solid haul comes from Rep. Bruce BraleyBruce BraleyTrump: Ernst wanted 'more seasoning' before entertaining VP offer Criminal sentencing bill tests McConnell-Grassley relationship Trump's VP list shrinks MORE (Iowa), Democrats' likely nominee to replace retiring Sen. Tom HarkinTom HarkinGrassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream Do candidates care about our health or just how much it costs? MORE (D-Iowa). Braley brought in $1 million as well.

In most of these races there are no announced GOP candidates, giving Democrats a jump-start on fundraising. Rep. Shelley Moore CapitoShelley Moore CapitoRepublicans want to grease tracks for Trump Republicans face divisions over ObamaCare repeal High out-of-pocket prescription drug cost: the patient perspective MORE (W.Va.), the GOP front-runner to replace retiring Sen. Jay RockefellerJay RockefellerLobbying world Overnight Tech: Senators place holds on FCC commissioner Overnight Tech: Senate panel to vote on Dem FCC commissioner MORE (D-W.Va.) did have a strong haul for an inexpensive state, bringing in more than $900,000. Rep. Bill Cassidy (R-La.), who recently announced a bid against Landrieu, brought in $500,000 but has a strong $2.4 million warchest.

Pryor, Landrieu, Hagan, Begich and Baucus are likely the most vulnerable incumbents.