Two races which are of particular interest are Georgia, where Democrat Stacey Abrams is seeking to become the first black woman to become governor of a USA state, and Florida, where Democrat Andrew Gillum is locked in a bitter battle with Republican and Trump acolyte Ron DeSantis.
The Republican Party now controls the chamber with a 43-seat majority, but it is widely expected that the Democrats will gain control in tomorrow's election.
The president said the main focus of his campaigning is on the Senate, while talking to reporters on the White House lawn Sunday.
Two polls released Sunday show Democrats with a modest but not comfortable lead in voter preference for which party controls Congress.
The gender gap cuts across lines of race and education, with non-white women (79% favor Democrats) and white women with college degrees (68% back the Democrat) breaking most heavily for the Democrats, while white men (57% Republican) and particularly white men without college degrees (65% back the Republican) are most deeply behind the GOP.
President Trump dismissed the idea of the so called "blue wave" yet again in the face of his economic success.
A fanatical Trump supporter was also arrested on charges of mailing pipe bombs to prominent opponents of the president, including Obama.
"Americans are making more money".
Likewise, the Republican National Committee said the GOP can preserve its House majority, arguing voters won't cede 23 seats or more seats to Democrats while the economy roars to life.
"It's all about turnout", said Senator Chris Van Hollen, as Democrats wage what polls say is an uphill battle to win control of the Senate.
Con esperanzas Puebla y Chivas para acceder a Liguilla
- Los tres puntos serán vitales para Puebla y Chivas , pero sólo uno podría llevárselos y seguir con la esperanza de una hipotética calificación a la " Fiesta Grande " del futbol mexicano.
In Texas, early voting has exceeded the entire turnout in 2014.
He says, "We should vote people in and out, not shout them in and out".
The companies are offering paid time-off and other schemes to give employees a better chance to vote.
The House of Representatives is the lower chamber of the US Congress and is made up of 435 seats.
What is at stake in these elections?And while Obama didn't break with his habit of refraining from calling out his successor by name, he also didn'tt hold back his criticism of the current administration.
In traditionally red Texas, popular Democrat Beto O'Rourke is trying to dethrone Senator Ted Cruz, while Republican Pete Stauber might flip a House Democratic stronghold in Minnesota.
In Florida and Georgia, Democrats are aiming to become the states' first African American governors. But a healthy economy favors the incumbent - and the United States economy has been growing with rare vigor.
Republicans are favoured to retain their slight majority in the U.S. Senate, now at two seats, which would let them retain the power to approve U.S. Supreme Court and other judicial nominations on straight party-line votes. Some are considered vulnerable, as they are in states won by Mr Trump when he was elected in 2016.
But even losing the House would be a blow for Mr Trump, as the situation would create a potential for legislative gridlock and even government shutdown.