With polls showing a number of must-win contests across the nation too close to call, Republicans are fighting hard to win over voters in swing states where GOP presidential nominee Donald Trump's tough rhetoric against Muslims, blacks, women and Hispanics could cost them votes. Victories in only a handful of states in November could flip the Senate and put Democrats back in control.

There are 34 seats up for grabs across the nation this fall, and Republicans control 24 of them right now. But Democrats have to win just four to six seats to take back the Senate while also defending their territory in other states, including Nevada, where Senate Minority Leader Harry Reid is leaving his seat open so he can step down and start his retirement. They will have to do that with Hillary Clinton, who faces questions about her use of a private email server during her stint as secretary of state, at the top of their ticket.

“The now-tight presidential race suggests that perhaps Clinton could pull off a narrow victory that still allows the GOP to hold the Senate,” wrote Larry Sabato, Kyle Kondik and Geoffrey Skelley of the University of Virginia Center for Politics on Monday.

The New York Times' The Upshot blog also declared this week that Republicans are slightly better positioned to maintain the Senate majority over Democrats at 53 percent to 47 percent. But a FiveThirtyEight forecast called the Senate election a tossup, with Democrats enjoying a slight lead.

Confused about what happens next? Here are the five Senate races to pay attention to heading into Election Day:

In Nevada, Republican US. Rep. Joe Heck faces Democrat Catherine Cortez Masto, a former Nevada attorney general. Recent polls show Heck slightly ahead of Cortez Mastro. In fact, he has come out on top in every survey since May. But Hispanic voters eager to keep Trump out of the White House could give her a victory in the swing state come November. Trump has said he will ban Muslims from entering the country, called Mexicans rapists and vowed to deport undocumented immigrants.

In Wisconsin, former Democratic Sen. Russ Feingold is battling Republican incumbent Sen. Ron Johnson, who beat Feingold in 2010. Polls put Feingold ahead this year, while a high presidential-year turnout could further fuel his campaign. A recent Monmouth University poll gave Feingold a double-digit lead against Johnson, but a Marquette University Law School poll had Feingold with only a 4-point advantage.

In Indiana, former Democratic Sen. Evan Bayh is taking on U.S. Rep. Todd Young. Bayah has a slight edge in the polls to replace retiring Republican Sen. Dan Coats, according to Real Clear Politics. Meanwhile, a poll from WTHR and Howey Politics, the most widely read political blog in the state, gave Bayah the edge with only 4 points earlier this month.

In Illinois, GOP Sen. Mark Kirk, who opposes Trump, is up against Democratic Rep. Tammy Duckworth. The Democratic-leaning state could be a tough battle for Kirk, but the latest polls show Duckworth with only a 3.5 percent advantage, making the contest a true unknown until voters weigh in.

In Arizona, Republican Sen. John McCain is running against Democratic U.S. Rep. Ann Kirkpatrick. McCain has been saying for months that he is worried Trump will hurt him at the ballot with the Republican-leaning state's growing Hispanic and progressive voters. Hispanics make up about 22 percent of the state’s eligible voters, but a recent NBC/Marist poll had McCain up 19 points, making the race seem less competitive for Democrats.