Voters in eight states choose candidates in battle for U.S. Congress

SACRAMENTO, Calif./PRINCETON, N.J. (Reuters) - Establishment-backed Democrats muscled past liberal challengers in key New Jersey congressional races on Tuesday, results showed, as voters in eight U.S. states, including crucial California, chose candidates for November's midterm elections.

Democrats, buoyed by President Donald Trump's poor approval ratings and a string of victories in special elections, hope to gain a majority in the 435-seat U.S. House of Representatives by flipping 23 seats this fall. The party has targeted more than a dozen Republican-held seats in California and New Jersey alone.

"I don't think the Democrats can take the House without New Jersey," said Brigid Harrison, a political science professor at Montclair State University.

Control of the House would likely derail or stall much of Republican President Donald Trump’s policy agenda, while ushering a new era of committee oversight and investigation into the administration.

A power shift would also lend strength to any potential effort to impeach Trump if Democrats decide grounds exist, either by virtue of the ongoing probe of Russia election interference by special counsel Robert Mueller or other reasons.

In southern New Jersey, where the retirement of a longtime Republican incumbent gave Democrats an opening, state Sen. Jeff Van Drew, a conservative Democrat, appeared set to prevail over left-leaning candidates frustrated by his past votes against same-sex marriage and minimum wage increases.

His Republican opponent was not yet clear late on Tuesday, with several candidates splitting the vote.

In a suburban New Jersey district, former Obama official Tom Malinowski was well ahead of two Democrats backed by independent liberal groups and will take on the moderate Republican Leonard Lance in what is expected to be a competitive race.

Democrats also hope to capture two other Republican-held seats in the state.

Like New Jersey, California is seen as key to the eventual outcome in November. Democrats have taken aim at 10 of the 14 House seats held by Republicans.

Tuesday's contests will determine whether Democratic candidates in the swing districts are moderate or more liberal, as well as whether they are shut out of some races altogether.

The state's so-called jungle primary system allows the two highest vote-getters to advance to the general election, regardless of party.

The battle for the top spots is particularly dramatic in the 48th Congressional District, where voters chose Democrat Hillary Clinton over Trump in the 2016 presidential election but re-elected longtime Republican Representative Dana Rohrabacher.

So many Democrats are vying to challenge Rohrabacher that there is a chance they will divide their party's vote, setting up a general election fight between the incumbent and a leading Republican opponent, Scott Baugh.

In California's gubernatorial primary, Democratic Lieutenant Governor Gavin Newsom is considered the front-runner in a 27-candidate race for an open seat.

If Republican John Cox wins the second spot on the ballot, his presence in the race could spur Republican turnout in November and possibly improve the party's chances in congressional races.

But if Democrat Antonio Villaraigosa, a former mayor of Los Angeles, makes the final cut, Republican leaders worry more of their voters will stay home in November, giving Democrats a better chance to flip some seats.

Trump in a series of Tuesday tweets backed Republican candidates.

"In High Tax, High Crime California, be sure to get out and vote for Republican John Cox for Governor. He will make a BIG difference!" Trump tweeted.

Republicans control both chambers of Congress. While Democrats need only a net total of two seats to take command of the Senate, Democratic senators must defend a bevy of seats in rural states such as Indiana, Montana, North Dakota and West Virginia that supported Trump two years ago.

Voters on Tuesday also went to the polls to choose candidates in primaries in Iowa, Mississippi, Alabama, Montana, South Dakota and New Mexico.

(Reporting by Sharon Bernstein in Sacramento, California, and Joseph Ax in Princeton, New Jersey; Additional reporting by James Oliphant; Editing by Colleen Jenkins and Cynthia Osterman)