Rep. Martha McSally, R-Ariz Bill Clark | CQ-Roll Call Group | Getty Images

As November's midterm elections draw closer, two of the most pivotal states in the fight for control of Congress hold primary elections Tuesday. Voters in Arizona and Florida choose nominees for two of the country's most important Senate races, along with a handful of highly competitive House elections. Democrats will vie to challenge GOP Gov. Doug Ducey in Arizona, while both major parties will jostle to replace Rick Scott, Florida's Republican governor and a Senate candidate. In Oklahoma, a primary runoff will determine the Republican contender in the race for governor. After Tuesday's elections, only a few more primaries — mostly in small, blue-leaning Northeastern states — will take place before November. Democrats are trying to flip 23 GOP-held seats to win a majority in the House. Meanwhile, the GOP faces a favorable map as it tries to keep or expand its 51-49 seat majority in the Senate.

Here are notable races to watch on Tuesday:

Arizona

The contest to replace retiring Republican Sen. Jeff Flake is Arizona's most high-profile this year. Democratic Rep. Kyrsten Sinema is all but assured to earn her party's nomination on Tuesday. The Republican side has proven a bit more unpredictable. The three candidates — Rep. Martha McSally, former state Sen. Kelli Ward and ex-Maricopa County Sheriff Joe Arpaio — have all tried to position themselves as the biggest supporter of Trump. That puts them in contrast to Flake, who has criticized Trump more frequently than most of his Senate GOP colleagues. McSally likely has an edge, as Ward and Arpaio could divide the state's more conservative voters. Trump has not yet endorsed a candidate in the race. But he has a relationship with Arpaio, whom he pardoned following a criminal contempt conviction related to his treatment of undocumented immigrants. Read more about the Arizona Senate race here. On the House side, Democrats covet the 2nd District seat McSally vacated. Former U.S. Rep. Ann Kirkpatrick has emerged as the top fundraiser and contender for the Democratic spot in the district, which leans slightly Republican relative to the rest of the country. Businesswoman Lea Marquez Peterson is considered the GOP frontrunner for the nomination. Republicans also have a chance to pick up the Democratic-held 1st District, though prognosticators give Rep. Tom O'Halleran a good shot at keeping his seat. State Rep. Steve Smith, businesswoman Tiffany Shedd and Air Force veteran Wendy Rogers aim to challenge O'Halleran. In the race for governor, former education official David Garcia and state Sen. Steve Farley appear to be the leading Democratic candidates to challenge Ducey.

Florida

Florida Governor Rick Scott greets people as he makes a campaign stop at Chico's Cuban Restaurant where he received an endorsement from the Florida Police Chiefs Association on June 14, 2018 in Hialeah, Florida. Joe Raedle | Getty Images

Florida's closely watched Senate race will be one of the most important in determining the Senate's partisan makeup next year. Democratic Sen. Bill Nelson faces a strong challenge from Scott, who has piled his own fortune into his campaign. Both Nelson and Scott are expected to breeze to their parties' nominations on Tuesday in the year's most expensive race. Read more about the Florida Senate contest here. The primary contests in the race to replace Scott as governor should prove more exciting on Tuesday. On the Republican side, Trump has put his weight behind Rep. Ron DeSantis, a conservative who has proudly tied himself to the president. The endorsement has given DeSantis a major boost as he tries to defeat the more traditional GOP alternative in state Agriculture Commissioner Adam Putnam. Former U.S. Rep. Gwen Graham appears to have a slight edge in a crowded Democratic primary field. Miami Beach Mayor Philip Levine is among the other contenders. Numerous House races in Florida will also be intriguing either on Tuesday or in November. A few Democratic House members from Florida face primary challenges. Former Jacksonville Mayor Alvin Brown hopes to take out Rep. Al Lawson in the sprawling 5th District, which stretches from Tallahassee to Jacksonville. Rep. Stephanie Murphy faces a challenge from lawyer Chardo Richardson in the 7th District outside of Orlando. Former U.S. Rep. Alan Grayson also hopes to unseat Rep. Darren Soto in the 9th District south of Orlando.

Republicans will compete to replace outgoing incumbents in three seats considered at least relatively safe for the GOP: DeSantis' 6th District, the 15th District represented by Rep. Dennis Ross and the 17th District held by Rep. Tom Rooney.

In the Miami-area 27th District, Democrats have their best pickup opportunity in the state as longtime GOP Rep. Ileana Ros-Lehtinen retires from a district that Democrat Hillary Clinton won by about 20 points in 2016. Former Health and Human Services Secretary Donna Shalala appears to lead the Democratic field in Tuesday's primary, while journalist Maria Elvira Salazar looks to have an edge on the Republican side.

At least one other Republican House member could face a tough challenge in November. Democrat Debbie Mucarsel-Powell is considered the frontrunner in the her party's primary to challenge GOP Rep. Carlos Curbelo in the 26th District, which encompasses the southern most parts of the state. Clinton also won Curbelo's district in 2016.

Oklahoma