Republicans hold a narrow majority in the Senate and can’t afford to give up too many Senate seats if they want to hold onto that power.
Republicans take up 51 seats in the Senate compared to 49 controlled by Democrats – including two independents who caucus with the Democratic Party.
This year, there are 35 seats up for grabs with five of them being ranked toss-up elections by Fox News. There are 19 seats that will likely remain Democrat compared to five that will likely remain Republican. Additionally, there are three seats that lean Democrat and three seats that lean Republican.
The map of Senate seats up for grabs is "much more favorable for Republicans," John Dinan, a professor of politics and international affairs at Wake Forest University, told Fox News.
"In a tough year for Republicans in the House, they have a gift in the map they've been given for the Senate," Dinan said, adding that "it's an uphill climb for Democrats" seeking to take control.
The toss-up races are considered to be in Nevada with incumbent GOP Sen. Dean Heller and Democrat Jacky Rosen; Arizona with GOP Rep. Martha McSally and Democratic Rep. Kyrsten Sinema; Missouri with incumbent Democratic Sen. Claire McCaskill and Republican Josh Hawley; Florida with incumbent Democratic Sen. Bill Nelson and GOP Gov. Rick Scott; and Indiana with incumbent Democratic Sen. Joe Donnelly and Republican Mike Braun.
The Montana Senate race between incumbent Democrat Jon Tester and Republican Matt Rosendale is seen as leaning Democrat, as well as the West Virginia Senate race between incumbent Democrat Joe Manchin and Republican Patrick Morrisey. And New Jersey Sen. Bob Menendez's re-election race against Republican Bob Hugin is also seen as leaning Democrat.
Fox News has ranked the four-way Mississippi special election as leaning Republican. The Tennessee race between Republican Rep. Marsha Blackburn and Democrat Phil Bredesen is also considered to be leaning Republican. Additionally, Texas Sen. Ted Cruz's race against Democrat Beto O'Rourke is ranked as leaning Republican.
Meanwhile, Senate races in Mississippi, Nebraska, North Dakota, Utah and Wyoming are considered likely Republican. Races in California, Connecticut, Delaware, Hawaii, Maine, Maryland, Massachusetts, Michigan, Minnesota, New Mexico, New York, Ohio, Pennsylvania, Rhode Island, Vermont, Virginia, Washington and Wisconsin are ranked likely Democrat.
Dinan said it's important to look to what topics these close Senate races are focusing on. For Republicans, candidates are harping on newly-confirmed Supreme Court Justice Brett Kavanaugh and immigration. For Democrats, the topic of health care seems to be one that drives their platform.
In more conservative states, vulnerable Democrats are looking to show the electorate they can be supportive of President Trump's agenda.
"If you look at states like Indiana, Montana or West Virginia, it's a race among Democratic candidates to show the electorate how they supported President Trump," Dinan said. "This is not what you see in some districts in the northeast or Pacific northwest in House races."