(CNN) If Democrats are going to have a realistic shot of winning the Senate, they will have to win the races in two of the states with primaries on Tuesday: Arizona and Florida. If they lose either one, their chance to win the Senate goes down precipitously.

Democrats have an uphill battle to take back the Senate in November. They need a net pickup of two seats, and only nine of the 35 seats up for election in 2018 are currently held by Republicans. Ten of the Democrats (or independents who caucus with the Democrats) occupy seats in states President Donald Trump won in 2016.

Arizona ranks as the Democrats' top pickup opportunity in 2018. It is one of only two seats with Republican senators and taking place in states that Trump won by fewer than 5 points or lost. It is the only state that meets this description where no incumbent is running for re-election. (Sen. Jeff Flake is retiring.)

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Arizona is also a state that may be becoming more purple. It hasn't had a Democratic senator since Dennis DeConcini left the chamber in 1995. Yet Arizona was one of the few states where Trump's 2016 margin was smaller than Mitt Romney's was in 2012.

Likely Democratic nominee Kyrsten Sinema looks to take full advantage of Democratic gains. Sinema, who is the second most moderate Democrat in the House, all but cleared the field upon announcing her entry into the race last year.

Her probable opponent is Republican Rep. Martha McSally, who also has a fairly moderate voting record. McSally, though, has had to fight her way through a tough three-way primary against the more conservative Kelli Ward and Joe Arpaio. The latest polling shows a clear McSally advantage in the primary, which no doubt pleases those who want a Republican to win in the fall. Her primary battle, however, may have cost her. McSally has had to adopt some more conservative and Trump-friendly rhetoric.

Sinema has held a consistent general-election lead over McSally in the area of 5 to 10 percentage points. This same polling has both candidates under 50%. That could give McSally room to grow, once the general election campaign begins.

Still, Sinema is a favorite for the fall campaign. Candidates in her polling position win about 75% of the time. That's necessary good news for Democratic Senate hopes, though far from sufficient. Without winning in Arizona, Democrats would need to win in Tennessee (where Trump won by 26 points in 2016) or Texas (where Republican Sen. Ted Cruz continues to be ahead of Democratic wunderkind Beto O'Rourke). They'd, of course, also need to win in Nevada, where Trump lost by 2 points, but where Republican Sen. Dean Heller has managed to stay neck and neck with Democrat Jacky Rosen.

The Florida Senate race is something altogether different.

Normally, senators like Democrat Bill Nelson don't lose re-election. In the over 110 times since 1982 when an incumbent senator of the opposition party has run in a midterm, the incumbent has lost only four times. Three of those four times were in years (1998 and 2002) when the president had an approval rating north of 60%. Trump's approval rating is in the low 40s. And all of those four occurrences happened in states that were reliable votes for the president's party in presidential elections. Florida is a swing state, on the other hand.

Even when Republican Gov. Rick Scott entered the race, it looked like it was going to be a tough road for Republicans. Scott had barely won his two elections in 2010 and 2014, and both of those took place in very good Republican years.

In other words, this should have been a race that Nelson won fairly easily. That has not been the case.

Scott has led in most of the recent polling by low single digits. Florida insiders too believe that Scott is a small favorite over Nelson.

Just what the heck has happened? Two key factors are working in Scott's favor.

First, he has raised a lot more money than Nelson . This has allowed him to spend a lot more on the airwaves.

Second, Scott's simply more popular than he was during his two runs for governor. Thanks in part to his actions after Hurricane Irma, his approval rating rose significantly . For the first time in his governorship, Scott has seen an extended period of positive net approval (approval minus disapproval) ratings.

It's far too early to count Nelson out, though. The fundamentals of this race still favor him.

Most of the polls that have given Scott a lead have been of lower quality (i.e. non-live interviewer or not transparent about their data collection), and Nelson had a small lead in earlier polls of higher quality.

Further, Nelson actually has a cash-on-hand advantage, though Scott has personal wealth he can spend. Still, you can be sure national Democrats will be spending a ton on the race in the final two months of the campaign.

Democrats, simply put, cannot afford to not put in a vigorous effort in Florida. Like with Arizona, Democrats need every win they can get, and this race on paper shouldn't be as difficult as other states. Democratic incumbents in Indiana, Missouri and North Dakota face much tougher electoral winds.

A loss in Florida, like in Arizona, would make a tough Senate map nearly impossible for Democrats.