If the Dems’ KS gambit wasn’t tricky enough for you, how about a sudden assault on the South Dakota seat being vacated by retiring Democrat Tim Johnson, which the Republicans have been taking for granted as theirs?
Here’s how Alex Altman explains it at Time:

If Democrats hold the Senate in November, it could be due to surprising success in states the party never expected to be competitive.
Just weeks after independent candidate Greg Orman surged in a Kansas Senate race that had been chalked into the Republican column, Democrats have spied another unlikely opening on the prairie. The Democratic Senatorial Campaign Committee (DSCC) on Wednesday pumped $1 million into the surprisingly competitive South Dakota Senate race.
The DSCC poured in the cash just hours after a new poll showed Republican Mike Rounds locked in a three-way contest with independent candidate Larry Pressler and Democrat Rick Weiland. The SurveyUSA poll released Wednesday showed Rounds with 35%, Pressler with 32% and Weiland with 28%. The race also includes a Tea Party candidate who could siphon votes from the GOP frontrunner.

Of course the GOP establishment is making dismissive comments. But rest assured they will be forced to spend lots of the party’s campaign cash and scarce GOTV resources to win the seat. If the polling margins narrow even a little, a muscular ground game could win it for Dems.
As for the reasons behind the Republican candidate’s tumble in the polls, Altman adds:

…Rounds, a former governor, has been dogged by a controversy over the state’s EB-5 program, a federal visa program that grants green cards to wealthy immigrants who invest at least $500,000 in economic development project. As governor, Rounds was a booster of the program, which has drawn criticism for mismanagement and lack of transparency after it was privatized by one of the governor’s allies.

It’s another indication that the Democratic Party is alert and agile, heading into the final month of the midterm campaign. Regardless of the outcome of the midterm elections, no one can fairly say that Democrats were caught napping anywhere in the Senate battlefield.
Ed Kilgore sums up the SD wrinkle at the Washington Monthly:

Now the SUSA poll could be an outlier, and I suspect the NRSC will be sending a lot of emergency help to Rounds while privately cursing him for somehow blowing a huge lead (controversy over Rounds’ support for and his brother’s role in the federal EB-5 visa program, which gives families of foreign investors a big advantage in applying for citizenship, has been a factor in his lagging poll numbers). But you can’t help but wonder what the evening of November 4 will be like if not only we are waiting for the usual insanely slow count from Alaska, and possibly two runoffs (LA and GA), but also not one but two indie senators who may or may not act in concert (and/or with Angus King). That’s some serious uncertainty, folks.

And in a year that was supposed to include a cakewalk Senate takeover for Republicans, more uncertainty cuts into their edge in a big way.