The chart above is from talkingpointsmemo.com. It shows the campaign money situation in the most competitive U.S. Senate races in 2018 -- the money raised in the first quarter, the cash on hand, and the amount spent so far.
The good news is that Democrats are not having much trouble raising campaign funds this year. Most of them raised more in the first quarter than their Republican opponents did, and most have more money on hand. That means they will likely have the funds to run a vigorous campaign -- at least as vigorous as their Republican opponents. That's a nice change, since far to often in the past they have been underfunded.
The bad news is that the Democrats must defend 23 seats, while the Republicans are only defending 8 seats. That means the Democrats will have to hold all their seats and then take 3 of the 8 Republican seats to take control of the Senate. And if the Democrats lose one or two seats in some red states (like West Virginia or North Dakota), then they will have to win 4 or 5 of the 8 GOP seats.
It can be done, but it will not be easy. Democrats will have to turn out in very large numbers to accomplish it.
If the Democrats fall short in 2018 in the Senate, the further good news is that they will have an even better chance in 2020. In 2020, the Republicans will have to defend 20 seats while the Democrats will need to defend only 11 seats. And those Republicans will have to run with a very unpopular and flawed presidential candidate at the top of their ticket.