At present Republicans dominate the Senate with 54 seats compared to the Democrat's 44 and another two held by independents. They also control the House of Representatives with majority of 247 to 188.
While Democrats are unlikely to gain control of the House, they could secure a 50/50 split in the Senate, according to University of Birmingham US politics Professor Scott Lucas.
"In practice, the casting vote is the presiding officer, who is the Vice President ... That's why the presidential race is especially important," he said, adding that it will also affect the appointment of a Supreme Court judge due to be made in 2017.
"If Clinton becomes President, I anticipate another [two judges] will retire. They would rather retire while a Democrat is in office. You can see why these appointments are going to be critical."
Specific ballot initiatives that will be voted on will change in each state.
California voters will choose from 17 statewide measures including Proposition 60 that allows them to decide whether porn stars should have to wear condoms while filming sex scenes.
They will also have to decide on Proposition 64, which would make recreational marijuana legal for those aged 21 or over and impose taxes on sales and cultivation in what is being hailed as a "watershed" for marijuana use across the country.
Another initiative would ban shops from giving out single-use plastic bags. One would repeal the death penalty in favour of life without parole, while a separate proposition would impose a time-frame on challenges for those on death row.
Recreational or medical marijuana measures will be on the ballot in Arizona, Arkansas, Florida, Maine, Massachusetts, Montana, Nevada and North Dakota.
California and Washington will ask voters whether they want gun owners to undergo background checks in a law that could enforce what President Obama has failed to push through Congress.
Arizona, Colorado, and Washington will vote on minimum wage while several more states will vote on healthcare measures.
Prof Lucas said the last 24 hours of the race have seen polls remain tight despite the FBI clearing Clinton from the emails found on Anthony Weiner's computer.
"She was looking very shaken up until 48 hours ago, but has not reversed the gains he has made in the last two weeks."