US voters will soon render their verdict on Donald Trump and his ruling Republican Party after two tumultuous years that have deeply shaken the world's oldest existing democracy.

Have Americans had enough? The answer comes at polling stations one month from Saturday, with Republicans battling to keep control of the US Congress and Democrats hoping a "blue wave" will wash them out.

Trump - among the most polarizing US leaders ever - is not on the ballot, but his presence looms large over the midterms, which he has declared "the most important congressional elections of our lifetime."

He has hit the campaign trail for political allies and to rally his base, attacking "obstructionist" Democrats and touting his success in improving the US economy.

"I'm not on the ticket," he told supporters at a Mississippi rally Tuesday. "But I am on the ticket, because this is also a referendum about me."

Some Republicans, especially those who prevailed in 2016 in districts won by Hillary Clinton, are running at arm's length from Trump, whose poll ratings remain under water. But many candidates have wholeheartedly embraced the president and acknowledged their election fortunes will rise or fall with him.

'Defining moment'

Democrats are energized, with well-funded campaigns aiming to snatch seats in Tennessee and other red states like Georgia, Kansas and Texas as they aim to reclaim the 435-member House of Representatives and the 100-member Senate. "Folks are sensing a sea change coming here," Tariq Thowfeek of the Texas Democratic Party told AFP in Austin.

He pointed to the compelling grassroots candidacy of Beto O'Rourke, whose bid to oust Republican Senator Ted Cruz would send shockwaves through the nation if it succeeds on November 6.

O'Rourke often speaks of this being "a defining moment for our country," and embraces the opportunity to engage with Texans of all stripes.

He drills down on kitchen table issues including health care - a top priority for Democratic voters - jobs and education, while demanding criminal justice reform, more rational immigration policies and gun safety legislation. But the man in the White House is still expected to have the predominant impact on the midterms.

"For Democrats the top issue by a mile is Donald Trump," said Larry Sabato, director of the University of Virginia's Center for Politics.

"He is the reason they're turning out to vote, and almost unanimously they're voting against him."

In the first midterms of the anti-harassment #MeToo era, the power of the female vote is certain to be tested - even more so with the dramatic debate currently roiling the US Senate.

The election season is occurring in the shadow of a bitter Supreme Court battle, in which Democrats are aiming to spoil the nomination of Brett Kavanaugh, a federal judge who stands accused of sexual assault.

'Rallying cry'

It remains unclear just how that partisan fight might impact the midterms. But Trump is confident his nominee, whose confirmation would shift the nine-justice high court rightward, will energize his base - a necessity if the party wants to hold ground in November.

Kavanaugh "is like a rallying cry for the Republicans," Trump told reporters Tuesday.

If Kavanaugh gets confirmed, it would mean Republicans control the White House, both chambers of Congress, and a conservative-leaning Supreme Court, potentially diminishing the sense of urgency among Republican voters.

Depressed turnout could spell disaster for Republicans, who face a hostile election climate.

Democrats must gain 23 seats to retake the House. Dave Wasserman of the non-partisan Cook Political Report predicts Democrats will flip 25 to 40 seats, while the Center for Politics says that number could near 50.

In a sign of 2018's Democratic tilt, a hefty 29 Republican-held House seats are rated a "toss up" by Cook, compared to just two Democratic seats.

"It's getting ugly" for Republicans, tweeted Wasserman, as he pointed to the GOP closing the funding spigot for some embattled candidates in districts won by Trump in 2016.

Trump has employed fear-mongering to counter the Democrats' enthusiasm edge.

"If Democrats get control, they will raise your taxes, flood your streets with criminal aliens, weaken our military, outlaw private health insurance and replace freedom with socialism," Trump said in Mississippi.–AFP

Meanwhile Democrats are promoting a diverse collection of candidates. Native Americans, Muslims, and Latinas are seeking to make history in many races, while Florida, Georgia and Maryland could elect their first black governors.

A new Quinnipiac Poll gives conservatives some hope, as it showed Republicans cutting the general Democratic lead in House races from 14 points to seven. In the Senate, Democrats have a steeper climb, even though Republicans have a slim 51-49 majority.

Of the 35 Senate seats in contention in November, Democrats are defending 26 of them - including 10 in states Trump won. Republicans are defending just nine seats.–AFP