WASHINGTON - US voters began to cast their ballot for the midterm elections as the first poll stations opened Tuesday morning.

The race for the House is tight. Experts believed all remains uncertain in this hotly-contested race which could come down to the wire.

"The race is tightening in a highly polarized electorate," said Dan Mahaffee, senior vice president and director of policy at the Center for the Study of Congress and the Presidency.

According to the non-partisan Cook Political Report, in recent weeks, the Democratic party has made gains even in areas which lean toward Republicans while the Republican party has gained ground in rural areas.

"The House elections are competitive because the country is closely polarized between the two parties," said Darrell West, senior fellow with Brookings Institution.

"America still is a 50-50 nation between Republicans and Democrats. A number of recent campaigns have been very close and neither party has managed to score a knockout blow over the other," West said.

The immigration issue forms the core of Trump's strategy to gain support for the Republican party.

Many white working class voters, a large chunk of Trump's supporters, resent the decades-long influx of illegal immigrants into the United States, claiming they lower wages and create more competition.

In recent days, Trump has focused his campaign speeches on the issue at rallies in the city of Macon, Georgia, and Chattanooga, Tennessee.

But Trump's tactic could backfire, some analysts said.

"Democrats have gained in recent weeks because of a backlash to Trump's nativist appeals," said West.

"Instead of talking about the strong economy, which could appeal to a broad range of people, he has focused on immigration and race issues, which are very divisive. That has helped Democrats pull into the lead in several states," West said.

Experts also predicted that there will be political gridlock if the Democratic party wins the race because investigations into Trump will be possible.

Mahaffee said with the control of the House's oversight committee, Democrats will be able to launch investigations into the Trump administration.

"Political deadlock probably will take place if Democrats do well because they will stymie Trump's policy agenda and investigate his various activities," West said.

Christopher Galdieri, assistant professor at Saint Anselm College said that "if Democrats win control of the House, the Trump presidency is over in terms of conservative legislation."

The question then becomes whether a compromise is possible, said Galdieri, adding that Democratic control of one or both chambers will mean lots of investigations into Trump and his administration."