Donald Trump campaigns on Monday afternoon. ( Reuters: Jonathan Ernst )
Sitting US presidents have historically faced a backlash at the midterm elections.
Donald Trump's approval ratings are particularly low right now.
The elections will be held on Tuesday, local time.
Donald Trump has acknowledged his Republican Party is facing an uphill battle at the midterm elections this week, amid polling that suggests he could lose control of Congress.
In his final major campaign event before the elections, Mr Trump was quick to highlight the historical trend that the sitting president's party "doesn't win the midterms".
"But we gave this a very strong shot," he said during a speech in Washington DC on Monday afternoon.
He later described the votes that would take place on Tuesday, local time, as "the most important midterm, I believe, in our lifetime".
"We have to keep our Congress strong and free," he said.
Analysts believe the GOP will be swamped in many House races, and while losing its Senate majority also remains a possibility, the contests in that chamber are considered more competitive.
Midterm elections are typically difficult for whichever party the president of the day represents.
In all but three midterms since 1934, the president's party has lost House seats, although Senate trends are harder to predict.
Mr Trump's approval ratings have plummeted since he returned to the White House two years ago.
Polls suggest only about 37 per cent of Americans approve of Mr Trump, which would be the second-lowest rating heading into a midterm election of any US president since the end of World War II.
The Democrats need to retain all their Senate seats and flip four more if they are to seize control of the upper chamber of US politics on Tuesday.
Mr Trump has spent the past few weeks trying to shore up support in states which he has typically been able to rely on for strong support over the past decade.
One of them is Texas, where the Democrats have not won a Senate seat for more than 30 years.
However, many polls show the GOP candidate Ken Paxton trailing his Democrat rival James Talarico in the polls.
The city of San Antonio, in the state's south, normally leans blue, although those margins have been shrinking at recent elections.
Local woman Debra Montelongo is planning to vote but has not decided who she will support.
"I've already done research. I've just pulled everything and printed it," she said.
"So today I'm going to go through and read through it all and then make my decision.
"I think what is important is, of course, taxes and interest rates, gas. What's best for the people, not just what their platform is."
The Senate race in Texas is expected to be close. ( ABC News: Cameron Schwarz )
James Henson from the Texas Politics Project at the University of Texas has been following the trends closely. He described this Senate race as the most competitive statewide election in three decades.
"The climate is very unusual," he said. "It's a climate in which Republicans are on their heels.
"And I don't think it makes sense to necessarily predict that the state is 'turning blue', as the phrase is here … but Republicans are having to put up much more of an active fight than they've had to in the past."
Other tight Senate races are anticipated in Michigan, Alaska, Iowa, Kansas, Maine and Ohio. All but one of those seats are currently held by the Republicans.
