美国选民准备中期选举投票

A sharply divided U.S. electorate is voting Tuesday to elect a new Congress and to render a midterm verdict on President Donald Trump. The results could shift the balance of power in Washington and alter the next two years of Trump's presidency.

All 435 seats in the House of Representatives are at stake Tuesday, plus 35 of the 100 U.S. Senate seats and 36 of the 50 state governorships.

Public opinion polls and analysts suggest that opposition Democrats have an advantage in the battle for control of the House of Representatives. Democrats are favored to win more House seats than they currently have and they need an overall gain of 23 to retake the House majority.

Republicans are counting on President Trump to rally his supporters to help maintain their narrow 51 to 49 seat edge in the Senate. Of the 35 Senate seats at stake Tuesday, Democrats hold 26 and Republicans hold nine.

Immigration focus

Democrats are trying to hold 10 Senate seats in states where Trump prevailed in the 2016 election, including Tennessee.

Trump blasted Democrats over immigration during a recent rally in Chattanooga.

"Democrats want to invite caravan after caravan of illegal aliens to pour into our country. I don't think so," Trump said, invoking images of the caravan of Central American migrants moving through Mexico. "No nation can allow its borders to be overrun. And that is an invasion. I don't care what they say. I don't care what the fake media says. That is an invasion of our country."

Democrats are getting some high-profile campaigners to help them including former President Barack Obama, who rallied voters in his home state of Illinois and told them Trump's deployment of U.S. troops to the border in response to the caravan was a "political stunt."

Early turnout has been huge in several states, especially for a midterm election when total voter turnout often struggles to reach 40 percent of eligible voters.

Trump a central issue

Polls show Democrats are most concerned with health care and the economy, with Republicans focused on immigration.

But Brookings Institution expert John Hudak said it is also clear that Trump is a major issue for both parties this year.

"This is a president who wants this midterm to be a referendum on him, largely because he thinks his own popularity is so great that it will carry Republicans across the finish line," Hudak said.

But Trump is not only battling Democrats in this year's election, he is also battling history.

"The big picture is that midterm elections go against the president's party," noted John Fortier of the Bipartisan Policy Center in Washington. "I think there will be no difference here. The Democrats will do quite well in the House of Representatives, in the governorships and state legislatures."

University of Virginia analyst Kyle Kondik notes that in the 29 congressional midterm elections held since 1900, the president's party has lost House seats in all but three — 1934, 1998 and 2002.

Will Democrats turn out?

Historically, though, Republicans are more reliable voters in midterm elections.

Gallup pollster Frank Newport said that puts pressure on Democrats to make sure their supporters get out and vote.

"Under the expectation that Republican voters typically are more likely to turn out, can Democrats energize people who identify with the Democratic Party to turn out and vote for their candidates?" Newport said.

If Democrats win enough House seats to reclaim the majority, Trump would face a shift in the balance of power in Washington.

Democrats are hoping for a wave election that would bring them control of the House and gubernatorial victories in key states like Florida, Ohio and Wisconsin.

Republicans are counting on Trump's frenetic campaign pace in the final days to help them retain or even expand their narrow Senate majority.