An expensive and closely watched Senate race in battleground Florida is tight, while Democrats hold solid leads in their bids to take or hold seats in Arizona and Ohio, according to three NBC News/Marist polls released Tuesday.

All three races will play a major role in whether Republicans can keep or expand their 51 seat to 49 seat majority in the chamber after November's midterm elections. The races will help to determine which policies the Senate pursues next year and whether President Donald Trump can keep up his flurry of conservative judicial appointments.

In Florida, incumbent Democratic Sen. Bill Nelson garners 49 percent of support among registered voters, versus 45 percent for Republican Gov. Rick Scott, an NBC/Marist survey says. The race, one of 10 this year where a Democrat defends a seat in a state Trump won in 2016, has already attracted a whopping $9 million in outside spending, according to the Federal Election Commission.

Democrats face a daunting Senate map, as the party's senators and independents who caucus with them defend 26 seats. Only nine Republican-held spots in the Senate are up for grabs.

Democrats have perhaps their best chance to flip a seat in Arizona, a state Trump won by about 4 percentage points in 2016. Sen. Jeff Flake, R-Ariz., will not run for re-election there.